Riddle for the Ages



""There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy.""

- Hamlet

It is the nature of mysteries that people want to find answers, and since fiction is in the business of giving people what they want, just about any mystery introduced in a story is going to be resolved. Important character gets murdered? We're gonna find out who the culprit was. The Eiffel Tower disappears? We're gonna find out where it went and why. Someone blacks out and wakes up three days later wearing antique samurai armor and a pink cowboy hat? We're gonna find out what led them to that state of affairs. While mysteries might sometimes be left unanswered inadvertently (such as a series being abruptly canceled, or the writers simply forgetting the mystery exists), if the people in charge know what they're doing, for every mystery introduced there's going to be an answer introduced somewhere down the line.

Except when there's not. Sometimes writers will create a mystery but never have any intent of actually giving us an answer. Bob will wake up one day with his right leg amputated, or Alice will discover someone's rearranged all her furniture while she was out. They might brush these matters aside cavalierly, or they might become obsessed with discovering the truth, but eventually they come to realize that, if they're ever going to get an answer, it's not going to be any time soon, and write off their respective mysteries as "one of those things".

This might be done for comedic effect, as part of a Mind Screw, or just to reflect reality. After all, how many of us have had something weird happen in our lives but never discovered the "why" behind it all? Like all tropes this sort of mystery can be used to tell really good or really horrible stories depending on the writers' skills, though it's generally considered good form to, if you're introducing an answerless mystery, let your audience know they're not getting an answer. It's not quite fair to present viewers a mystery, suspects, and clues that clearly have some sort of resolution, if only in the author's head, but never say what it is. Make it clear that you're not going to let people in on an important secret that you or someone on the production staff knows the answer to, and years might go by and people will still be asking you, "Why did X commit Y on Z?"

Please note that, to keep the examples section from getting bogged down, we should list only cases where a mystery is left unanswered and, either through context within the story or via Word of God, we know it was never meant to receive an answer. Without either of these things, it should be assumed that the writers meant to answer the mystery, but have not yet gotten around to doing so.

Super-Trope of Maybe Magic, Maybe Mundane. Cousin of the Cryptic Background Reference (they both add to the sense of a larger universe by leaving unexplained mysteries). See The Un-Reveal. Compare Noodle Incident, The Unsolved Mystery, What Happened to the Mouse?, and some Big Lipped Alligators.

Not to be confused with A Riddle Wrapped in a Mystery Inside An Enigma, nor with Lord Voldemort.

Anime and Manga

 * In Maison Ikkoku, after fourteen volumes and some six years of in-universe time, neither the readers nor the other characters have any real idea of Mr. Yotsuya does for a living...or, indeed, any knowledge of his life outside the apartment. His one and only Day in The Limelight is devoted to him evading the other characters' attempts to figure out what he does all day.
 * An episode of Naruto is dedicated to Naruto, Sakura, and Sasuke trying to find out what Kakashi looks like underneath his mask. Neither they nor the audience ever get a look at it, but it can apparently make Even the Guys Want Him.
 * In Code Geass, CC tells Lelouch her real name, but the sound is muted the exact moment she says it, preventing the audience from finding out.
 * Baka to Test to Shoukanjuu: What is Hideyoshi's gender?
 * Here is a short list of questions that Neon Genesis Evangelion leave unanswered:
 * Who ?
 * What did ?
 * Which ending (TV or OVA) actually happened? Is the OVA ending taking place for real, or is it just what Shinji wanted out of Instrumentality?
 * Theoretically, these questions could be answered by the movie series.
 * Did Bradley from Fullmetal Alchemist

Comic Books

 * Seen in Tintin's adventure Land of Black Gold: Captain Haddock shows up to rescue Tintin even though he was half a world away, and never gets around to explaining how that was possible. He's interrupted right after "It's both very simple and very complicated."
 * The lack of explanation for Haddock's appearance is actually a meta joke: the original version of Land of Black Gold was initially serialized in a newspaper in 1939 and 1940, but after Germany occupied Belgium in 1940, Hergé thought that the comic would not pass the German censors because to its political nature, so the publication of Land of Black Gold was stopped mid-story. At this point Captain Haddock hadn't yet been introduced in Tintin, so naturally he didn't appear in Land of Black Gold either. Several other Tintin stories were published before Hergé decided to redraw Land of Black Gold in 1948, and in these intervening stories Haddock had become the most significant character in the series besides Tintin himself. Thus it would've been odd if Haddock had been left out of the new version of Land of Black Gold, but on the other hand he didn't really belong to a story that had been scripted before he even existed. This is the reason why Haddock is virtually absent from the story until the very end, and why there's no explanation for his sudden appearance. The lack of explanation is Hergé's comment on Haddock "invading" a story he wasn't originally a part of. So there is a solution to the mystery on a meta level, but not in the text.
 * In Gaston Lagaffe, what are those contracts about that M. De Mesmaeker is always trying to sign?
 * In Y: The Last Man, no one knows what caused the Gendercide. While several theories are made in the story, and Word of God confirms that one of those theories is true, he refuses to say which one.

Film

 * Used for purposes of Mind Screw in the French film The Moustache, which also has No Ending. The protagonist may or may not have Ripple-Effect-Proof Memory, with someone or something repeatedly changing the world around him. He also may or may not be the only one getting messed with, and it's left uncertain whether he'll ever figure out what's going on and how to end it.
 * The contents of the suitcase in Pulp Fiction.
 * The same is true of the silver case in Ronin.
 * Likewise, the contents of the unopened package in Cast Away. (When pumped for an answer, the creators have stated it was a solar operated waterproof satellite phone. That is the only answer they will ever give.)
 * In Cache, we never find out who sent the tapes, or why. There are many clues, but no answers.
 * What is the Backstory of the Joker? It's even a specific trope.
 * In Star Wars, the exact name and homeworld of Yoda's Species are still unknown.
 * Also, what jawas look like under their hoods.
 * In The Maltese Falcon we never do find out what happened to the original.
 * What's whispered at the end of Lost in Translation?
 * What does Laura whisper to Brenden at the end of Brick? It's in the subtitles.
 * In Fright Night, we never do learn what Billy Cole was, only that he was neither human nor vampire. The novelization shows the heroes are as perplexed about this as the reader.
 * The very last shot of Inception is.
 * In the Disney film Candleshoe, Jodie Foster's character is hired by a con artist to pose as the Long-Lost Relative of an English noblewoman.
 * In The Quiet Man Mary Kate whispers something to her husband Sean Thorton which gives him a shock. This is also a Real Life mystery, as director John Ford told Maureen O'Hara to whisper something shocking to John Wayne to get that reaction. The three of them never explained what was said... And unless O'Hara mentioned it to someone else for posterity, when she dies the secret dies with her.
 * The Hangover: What the hell happened to that chair?

Literature

 * Frank R. Stockton's "The Lady or the Tiger?" In the story, a king forces each criminal to choose one of two doors. Behind one is a beautiful maiden that he will instantly marry. Behind the other is a ravenous tiger that will tear him to pieces. One criminal is the lover of the princess, who knows which door leads where. She gestures toward a door, and he opens it, but what does he find? Could the princess bear to see him marry another woman, or would she rather he died? The story caused something of a sensation in the years following its release, with readers clamoring to wring an answer from the author. A common writing assignment in classrooms is to supply an answer to the question.
 * Alice in Wonderland:
 * "Why is a raven like a writing desk?" The riddle is never intended to have an answer, prompting many fans throughout the ages to supply their own answers:
 * Sam Loyd's "Because Poe wrote on both" is the most popular answer.
 * "Because the notes for which they are noted are not noted for being musical notes." Sam Loyd again.
 * Stephen King's "The Shining" has a rather good answer for that: "The higher the fewer, of course."
 * Carroll himself eventually supplied his own answer, but only after years of people asking: "Because it can produce a few notes, tho they are very flat; and it is nevar put with the wrong end in front." Note that "nevar" is "raven" spelled backwards.
 * "Because there is a B in both and an N in neither." (Aldous Huxley)
 * "Because it slopes with a flap." (Cyril Pearson)
 * One Two Lumps strip both re-uses the above "Poe wrote on both", and also suggests "Both have inky quills."
 * From Sir Apropos of Nothing, "Because it is only with quills that they truly take flight."
 * Another one: Was Through the Looking Glass Alice's dream, or the Red King's dream?
 * The Lord of the Rings: What is Tom Bombadil?
 * The Virgin Suicides: Why did the girls kill themselves?
 * The Hitchhiker's Guide to the Galaxy: What is the Ultimate Question of Life, the Universe and Everything?
 * Simultaneously knowing both the Ultimate Question and the Ultimate Answer would cause the universe to rearrange itself into something even more incomprehensible. This may have happened already.
 * From The Wheel of Time, who killed was originally intended to be one of these, but due to fans constantly asking, it will be revealed in one of the last books.
 * And it was. In the glossary of the 13th book. For those who care, it was admitted to be.
 * A gag in More Information Than You Require involves Hodgman's encounter with director Peter Berg on a plane and finding out he had two copies of Dune with him on the plane, because he was planning on making a Dune movie. This does not explain why he had two copies with him. This becomes a brief running gag, in which Hodgman provides even less helpful explanations that still don't explain why he would have two copies of the book, leaving it a mystery to us all.
 * This is the whole point of Special Topics in Calamity Physics, which sets up dozens of mysteries and hardly answers any of them. At the very end of the book, the narrator gives a "quiz" in which she asks readers to come to their own conclusions on what happened. Ultimately, the implication is that it doesn't matter, because the narrator has moved on and grown through the experience.
 * Stanislaw Lem's Solaris: Why did the planet send the replicas of people? The main theme of the novel is that we can't find out, because humans can't comprehend a truly alien intelligence.
 * And they're not very good at understanding other humans either.
 * 1984 raises the question of whether Emmanuel Goldstein and his revolution against the Party actually exists, or if they're simply a fabrication the Party uses as a target for the population's hatred and as bait to weed out dissidents. O'Brien states quite firmly and adamantly that Winston will never learn the truth about this.
 * In On Stranger Tides, a pirate decides he'd rather be shot than hanged, and asks his Navy captors' leader an Armor-Piercing Question ("Is it true what Panda Beecher once told me about you?"), that implies the captain is either a smuggler or a sexual deviant. This provokes a fight in which the enraged captain is killed. After escaping, the pirate explains his ploy to his accomplice, and admits that he hadn't actually known anything about the captain: he just knew lots of Navy officers smuggled for Beecher or patronized Beecher's whorehouse, and they'll never know which offense the dead man assumed he'd been accused of.
 * In The Shining who is the Manager of the hotel that the ghosts refer to?
 * In The Tripods books, there are suggestions that the Sphere Chase is more than just a sport. Our heroes first become aware of it when they encounter two tripods practicing for it while on patrol. After Ruki is captured, he refuses to discuss the game, even though he does answer questions about the city and his race which are far more damaging.
 * Why did Dracula move to the US in the 1970s? This is a minor Running Gag in Fred Saberhagen's New Dracula series. Several times someone asks Dracula why he left Europe and he starts to reply with something like, "I came here because I like your--", only to be interrupted before he can finish by an American talking about something else.
 * In the Sherlock Holmes story The Five Orange Pips, while Holmes's explanation certainly makes sense, the mystery is never conclusively solved, as the primary suspects vanish without a trace with only vague hints as to their fate.
 * In David Copperfield,  swims out to sea to save a shipwrecked man from drowning. Shortly afterwards,
 * In Algernon Blackwood's "The Man Who Found Out", an explorer discovers the ancient Tablets of the Gods, reputed to explain the true purpose of human life. Once translated, their revelations cause him and the friend who inherits the Tablets to succumb to despair, then go to extreme lengths to conceal their contents: contents, that the reader never learns, apparently mercifully.
 * A Series of Unfortunate Events loves this. What were the exact circumstances surrounding the death of the children's parents? What was the cause of the schism in VFD? What was the exact cause of the death of Olaf's parents? (All we're told is that ). What happened to the people living on the island? Did they ? What happened to ? What is the Great Unknown? What was in the Sugar Bowl, exactly? What was the fate of the guests at the hotel? Who was the father of Kit Snickett's baby? That's not even half of them all.
 * There was also a scene on an island that everything washes up on eventually. They listed quite a few objects and vaguely hinted of the stories behind them.
 * Machado de Assis has a book that's been interpreted in different manners since 1899. The story is about a man that tells his life, his lofe for Capitu (his wife) and the doubts about her adultery. No one could ever tell if the protagonist was right, that his wife cheated him, or if the madness and the jealousy have taken their toll. Even today.
 * In The Princess Bride, William Goldman (who claims to be merely editing the original Morganstern) notes that the original manuscript ended on a 'Lady or the Tiger'-esque note, with Humperdinck and his cronies pursuing the four fugitives, who experience a number of setbacks. He further claims that it's left permanently open-ended as to whether or not they got away and lived Happily Ever After, but that as far as he's concerned, they did. Of course, Goldman is the actual author of the book, so the ending isn't really ambiguous.

Live Action TV
"George: Did you ever solve the riddle of the pie? Jerry: No, that's one for the ages."
 * Joss Whedon loves this, it seems.
 * In Buffy the Vampire Slayer, what was Buffy's heaven? Was "Normal Again" true? How did Warren learn how to build Ted-level robots? How did Anya cause the Russian Revolution? Exactly how close were Ethan and Ripper? What is the origin of Faith's tattoo? Explained in one of the books (it's the Mark of Kakistos, gotten while she was being possessed by a pissed off dead Slayer). Did the PTB extract Angel from that Hell Dimension? How did Angel get the Amulet with Spike in it? How did Buffy and the gang save the world and from what in "The Zeppo" (the episode focused on Xander and an unrelated event, so the audience only gets glimpses of what Buffy's doing)? Oh, there's so much more.
 * In Firefly, has River always been psychic and the Alliance just amplified it or was she just that smart? What's up with Inara and that syringe? (.) Why did that group only help Simon rescue River? Who was that group? What are the origins of the Academy? How is Blue Sun involved? Book's past?
 * Seinfeld is the Trope Namer with the episode "The Pie." The very first scene of the episode has Jerry offering his Girl of the Week a slice of apple pie, which she refuses to taste. She doesn't give a reason, just shakes her head, and Jerry becomes obsessed with figuring out why she won't try the pie. He rules out a lot of possibilities, but by episode's end, he's still as clueless as ever.

"Reporter #1: One minute to 8:00. Reporter #2: Is your watch right? [A noise is heard and everyone suddenly goes to investigate] Mike: (Somberly) Is his watch right? We may never know."
 * In "The Seven", Jerry dates a woman who always appears to be wearing the same dress. He wonders if she never changes her clothes, if she has a closet full of dresses like that, or if it's something else entirely. He never finds out and neither does the audience.
 * In the How I Met Your Mother episode "The Pineapple Incident," Ted gets drunk, blacks out, and wakes up the next morning with a sprained ankle, a burned coat, an unknown phone number written on his arm, a woman he doesn't know in his bed, and a pineapple on his nightstand. Through some detective work, Ted discovers exactly what he did while drunk to cause each of those things ... except the pineapple. Future Ted confirms that he never did find out where the pineapple came from. But it was delicious!
 * The woman, Trudy, makes a post on the website Ted Mosby is Not a Jerk. It doesn't reveal much about the pineapple, except for the fact that Ted probably shouldn't have eaten it.
 * In the episode "Zoo Or False," we don't find out how Marshall's wallet went missing. Nor do we find out whether Bobo really escaped from his cage, stole a doll and climbed Ted's model of the Empire State Building.
 * What does Barney do for a living? We know where he works, but not what he actually does. Even Marshall, after working with him for a time, doesn't know what he does. He changes the subject whenever he's asked about it directly.
 * In the Doctor Who non-canon episode "Curse of the Fatal Death" his companion frequently asks "why/how/what [whatever]?" and he always answers "I'll explain later," but the explanations never happen.
 * This is a reference to the many, many times he said "I'll explain later" or something similar on the show. There used to be a website that listed every instance, but it seems to have disappeared.
 * Specifically,
 * When killed himself on House, there was no buildup leading to his suicide, and in the aftermath no one was able to find any reason why he might have done it. Since House's main source of fun in life is finding answers to mysteries, he finds this extremely frustrating.
 * Also, no explanation was ever given, nor intended, for why in the second-season finale. Nor for what became of "Harpo" in the same episode.
 * Parodied in the Mystery Science Theater 3000 episode Prince of Space: Japanese reporters are awaiting the scheduled arrival of a space invasion:


 * In Twin Peaks, the question "Who killed Laura Palmer?" was intended to be one, but was forced to be resolved due to Executive Meddling.
 * On Top Gear, the real identity of The Stig. Or the age of The Stig. Or the origin of The Stig. Or pretty much anything about Stig beyond "tame racing driver."
 * Well, there was some suggestion that the Stig is in fact, but after  it seemed disproved.
 * Farscape: who killed Salis? Chiana, Durka or somebody else (there's a widespread Epileptic Trees theory that it was Zhaan).
 * In one episode of Boy Meets World the characters engage in an Escalating War and it's never explained how they pulled off certain outrageous pranks. Like, how the hell did Cory and Shawn move Rachel's car into her dorm room? And where did Rachel, Jack and Angela get that bear?
 * In the One Foot in the Grave episode "The Futility of the Fly", the Framing Story is that a playwright briefly employed by the Meldrews as a housekeeper has made a play about the bizarre things that occur to Victor in a typical day. One of these is being sent a giant plastic fly, and at the end, her backer objects that this never gets explained. She protests that it never did get explained, it was just this weird thing that happened and they never found out why, but he's not convinced.
 * What was in the Christmas present from her parents that Temperance finally opens at the end of a Bones Christmas Episode?

Music

 * In the comedy song "The Homecoming Queen's Got A Gun", we'll never know who Johnny was, because only Debbie knew and, like, she's dead.

Newspaper Comics

 * Calvin and Hobbes:
 * What was the Noodle Incident?
 * What is Calvin's family name?
 * What is the subject matter of Hamster Huey and the Gooey Kablooie?
 * What, precisely, is Hobbes—a stuffed animal or a living creature?
 * These are all questions that Bill Watterson has specifically refused to answer, either feeling the answers unimportant (as in the case with the characters' full names), or more satisfying if left up to the reader to decide (as is the case with Hobbes's true nature).
 * What exactly did Sally Forth's husband do for a living? Even they aren't sure.
 * Peanuts: Did Charlie Brown manage to kick the football for this one time?
 * When a Dilbert story arc involved the title character getting a girlfriend, the issue arose amongst the readers of whether Dilbert might actually "score" with her. Since sex wasn't something that could be openly discussed in a newspaper comic, Scott Adams told his readers that should Dilbert get lucky, his perpetually upturned tie would be drawn hanging flat. The flat tie strip did eventually come, but Adams still wrote the comic in such a way that it wasn't clear if Dilbert had had sex or not.

Opera

 * In Gilbert and Sullivan's The Yeomen of the Guard, the jester Jack Point asks the riddle "Can you tell me, sir, why a cook's brain-pan is like an overwound clock?" He never gets the chance to give the answer.
 * The Gilbert and Sullivan fan site savoy.net offers the answer: Because they are both so tight that they don't give seconds.

Religion and Mythology

 * Where was Jesus and what was he doing between the ages of twelve and thirty-ish? The Gospel writers were not the least bit interested in even hinting at it (save for a glib, hasty note about him "growing in wisdom and spirit", which tells us nothing specific).
 * It's all tied up with Jesus' family
 * He stared down a cave full of dragons, among other things.
 * What does the Sampo actually do?
 * What did Odin whisper to his late son Baldr's (the first Aesir ever to be Killed Off for Real) ear at his funeral?
 * Odin's two brothers, Vile/Honer and Ve, who helped him create the world and the creatures in it, simply disappear one day. Odin goes to great lengths to find the answer to their disappearance, finally sacrificing his one eye in the Well of Wisdom (as the eye can then see the past, present and future). Yet, he never tells a living soul the answer except his wife. He reasons that "What three know, the whole world knows."
 * When Baldr has been slain Odin rides to the death realm, overcoming many perils for twelve days in order to find him. Finally he meets an old witch. Having spoken with her Odin abandons his search and goes back to spend his days preparing for Ragnarok. He never reveals to anyone what the witch said.
 * The pronunciation of the name of G-d, i.e., the Tetragrammaton. The name was never written with the vowels because Biblical Hebrew was never written with vowels. As a result, we no longer know for certain how the true name of G-d was supposed to be pronounced.

Tabletop Games

 * What are the Dark Powers of Ravenloft, and what are they up to? Word of God from TSR's writers and canon statements from Arthaus proclaim that this will never be revealed, and a novel that dared to try it was declared non-canon for doing so.
 * What is the Lady of Pain of Planescape?

Theatre

 * In Waiting for Godot, who or what is Godot? Why are they waiting for him? Will he ever come? And just what on Earth was Lucky going on about?

Video Games

 * Will the convoluted timeline of The Legend of Zelda ever be fully comprehended by the fans? Is there even any sort of real chronology? Did Zelda actually change gender when she became Sheik, or did she just change her appearance?
 * Touhou has a crapload of this, probably as ZUN's way of encouraging fanwork. What happens if Rumia's ribbon is removed? What's the deal with Flandre's wings? What's Sakuya's backstory, and why was Eirin surprised to see her? Just how powerful would Alice be if she went all-out? What happened to Youki? Just how old is Eirin, anyway? What's Maribel's deal? Where exactly is Gensoukyou? How much of the PC-98 games are canon?
 * What is the name of the protagonist in Gothic? What was he imprisoned for?
 * Who exactly was the Head from the first Myth series game.
 * Pokémon: Just what the hell does the rest of Diglett and Dugtrio's bodies look like? And what do Cubone and Marowak's heads look like under the skull they wear?
 * Cubone's face has partially been revealed.
 * In Planescape: Torment, what did The Nameless One do that results in him ? For that matter, what is his name?
 * Will Shenmue ever be continued and Shenmue 2's massive game changing cliffhanger be answered?
 * Team Fortress 2. Who is the Pyro? What does he look like under his mask? What is he really saying? Is he really a woman? Some people hold out hope that this year's upcoming Meet the Pyro will explain everything, but knowing Valve, they'll likely stick to this trope.

Web Animation

 * Homestar Runner: How do you type with boxing gloves on?
 * Similarly, any information about the parents of any of the main characters save for Pom Pom and Homsar. Except Homsar claims to be the son of a cup of coffee and a chipwich, which may or may not be true.

Web Original

 * A good portion of the material covered by The Cinema Snob fall in this category in one way or another, whether due to their relative obscurity or the shady circumstances behind their creation. Among the more infamous though would be Bat Pussy, a 1970s pornographic "spoof" of which nothing is known about its production, let alone the people involved in it.

Webcomics

 * Sluggy Freelance - "Who murdered Sherrif Deer?"
 * Bob and George - How did Metool-D2 end up in the Alternate Universe? The characters know, but the audience never will!
 * Shortpacked contains a couple, the most glaring one being Galasso's resurrections of Ronald Reagan and Jesus Christ, to say nothing of that of Mike Warner. Resurrection tech exists in the comic's universe, but it explicitly requires nine months to work (Mike showed up after being dead for five) and the subject's DNA (all of Mike's was destroyed), and it's also under government lock and key with an extensive waiting list. There's also the matter of the Drama Tag, the pulling of which may or may not have resulted in a Cosmic Retcon of Amber's childhood if it even exists at all.

Western Animation
"Bart: That's impossible! How did you get here first? Mr. Burns: Oh, there'll be plenty of time for explanations later."
 * In The Simpsons
 * What state is Springfield in? It's clear that Springfield is not in any specific state and has geography suited to whatever the story requires, but that doesn't stop Lisa from asserting that you can figure it out if you look at the clues.
 * Matt Groening has stated it's in Oregon, but then again...
 * One episode has Bart and Lisa escape from Mr. Burns by dropping down a laundry chute he's too big to fit into. But when they land in his mansion's basement, he's already waiting there with a gun.

"Mr. Burns: Yes, I was a bit worried when he swallowed me, but ... you know the rest."
 * In another episode, having forgotten his name, Grandpa checks his underwear. When asked how he removed them without removing his pants, he shudders and admits, "I don't know!"
 * In another episode Mr. Burns has gone to Scotland to capture the Loch Ness monster, which swallows him. The next scene shows the monster hanging tied up under Burns's helicopter on their way back to the States. Smithers is impressed that Mr. Burns could subdue the monster.

"Raven: Face it, Red X could be anyone. Anyone smart enough to find the suit and dumb enough to take it for a joyride."
 * In Teen Titans the episode "X" involved Red X (a supervillain identity Robin had previously invented as part of a Reverse Mole plan) showing up, with someone we can be pretty darn sure is not Robin under the signature Red X suit. Fans have wondered for some time who the new Red X really is, and Beast Boy came up with a number of theories, but Word of God says Raven came to the only conclusion that mattered:

"Kuzco: I don't believe it! How did you get back here before us? Yzma: ...How did we get here, Kronk? Kronk: (pulls out a map) You got me. By all accounts, it doesn't make sense."
 * The most popular theory seems to be that he's Jason Todd, the second Robin.
 * The episode "Revved Up" revolved around Robin trying to get back a briefcase that Ding Dong Daddy had stolen from him. When asked what's inside it, Robin just says, "It's personal." When the briefcase is finally opened, its lid fills the entire screen, causing a Smash to Black, the end of the episode, and legions of fanboys/girls demanding to know, "What's in the frikkin' briefcase!!!"
 * The series finale revolved around one of these. Beast Boy sees a girl who looks exactly like Terra (his Love Interest from Season 2 who had been transformed into a statue). When he discovers that Terra's statue is missing, he assumes the girl he saw is Terra, but she protests, saying that her name isn't Terra, that she's never met Beast Boy before, and doesn't have any superpowers. Epileptic Trees abound, ranging from, "She's just pretending not to remember," to "She has amnesia," to "She's a clone," to "She's just a girl who really looks like Terra," to "Beast Boy's gone crazy and is imagining the whole thing." Since the episode's about letting go of the past and moving on, Beast Boy (and the audience) never get to discover the truth.
 * The answer is finally revealed to this one in the Teen Titans Go official tie-in comic series:
 * This is pointed out in The Emperors New Groove, when Yzma and Kronk manage to beat Kuzco and Pacha back to the castle, even after falling into a ravine.


 * Played straight and subverted in the "Riddler's Reform" episode of Batman: The Animated Series. When Batman is able to escape The Riddler's deathtrap of sealing him in a building that has high explosives in it, the Riddler is so bewildered by Batman's escape that he no longer cares about going to Arkham or losing his now legitimate business and lifestyle and only cares about finding out how Batman was able to survive a trap he clearly shouldn't have been able to. While Bruce later explains it to Dick Grayson, and the audience, back at Wayne Manor, he never explains it to the Riddler himself. Mr. Nygma ends up going even more insane due to not getting an explanation.
 * In-Universe example from Phineas and Ferb—where exactly do their projects disappear to every day? They even try to work it out in the episode "Don't Even Blink", but (un)fortunately the Doof was working on an invisibility/destruct-inator that day, so they never found out.
 * Avatar: The Last Airbender:
 * Is Ursa alive, and if so, where is she?
 * Who were Aang's parents?
 * Why do the Gan-Jins and Zhangs really hate each other so much?
 * Did Aang and Katara kiss in "The Cave of Two Lovers"?
 * Sequel Series The Legend of Korra is already being pretty sadistic about some of these. The first episode includes time-skipped Katara retelling her adventures to her grandchildren. One of them mentions that she's dying to know what happened to Zuko's mom. Katara assures them that it's an incredible tale, but is interrupted before she can even start.
 * SpongeBob SquarePants: What is the secret formula of the Krabby Patty?
 * Arthur: What happened to D. W.'s snowball?
 * Aliens took it thinking it was some sort of Earth delicacy. For once, Buster's alien theory is right.
 * Jimmy Two-Shoes: Where did Jimmy come from? What is the nature of Miseryville? Is it really Hell? Is it just a hallucination of Jimmy's? Is he dreaming?
 * Venture Bros: What did Rusty Venture do to make the Monarch hate him so much? Lampshaded in one episode when Tim-Tom and Kevin question Dr. Girlfriend about it, only to realise they're not going to get a straight answer from her.
 * At the end of the Kim Possible episode "Stop Team Go", Shego - who has been turned good due to the attitudinator - tells Kim that "I couldn't tell you this while I was evil, but -" and then her true personality is restored by the malfunctioning attitudinator, which is then crushed. Exactly what she was planning to tell Kim may never be known.

Real Life

 * Some mathematical questions, like the Continuum hypothesis: does a set exist which is uncountable, but of a lesser cardinality than the set of real numbers? It turns out that either "yes" or "no" is consistent with the set theory axioms, as long as the set theory itself is consistent.
 * What came first: chicken or egg? More of an historical example since, as The Other Wiki points out, the riddle was basically solved once evolution became understood. If you count non-chicken eggs, the egg easily wins. If you don't, then the first chicken egg was laid by something that, genetically, wasn't quite a chicken, so the first fully-chicken egg still came before the first full-chicken.
 * Unless a chicken egg is defined as either an egg laid by a chicken or an egg laid by a chicken as well as containing a chicken embryo. In both those cases, the chicken came first, having been born from a "not quite chicken" egg. Fun with semantics.
 * Due to loss or degradation of whatever evidence once existed, Jack the Ripper's identity will probably never be known.
 * This goes for any number of other unsolved murders, both modern and ancient. Lizzie Borden's parents' killer was never convicted, after all.
 * Advances in vat-grown meat technology mean that we will, in about twenty minutes, be able to grow meat from any animal. Those with a curious palate will be able to sample anything, without concern for cruelty or endangerment. Except extinct creatures, whose flavour will forever remain a mystery.
 * Or not. Then again, that only works for recently extinct species with sufficiently preserved remains.
 * Or perhaps we'll be able to find out if human flesh does taste like chicken.
 * Twenty-plus years after the fact, it seems unlikely we'll ever know the true nature or purpose of the man behind the Max Headroom broadcast intrusion.
 * Although we may now have a lead...