Chekhov's Classroom

"Pay attention in class, especially science class, because you will need the information later in the episode."

This trope is the awkward tendency of programs to use precisely what they learned in school (almost always science) earlier that episode, and to lampshade it through grating dialogue. Shows aimed at an older audience can make it slightly more subtle; even so, it usually comes out like this:

""Oh no! We're being attacked by Gef the Talking Mongoose. Wait a minute, say, Swotty McCliche, weren't you studying how to defeat talking mongooses just this morning?" "Oh yes, I totally forgot. How conveniently stupid of me!""

Common in Edutainment Shows. May be the payoff for a "Reading Is Cool" Aesop.

Compare Strange Minds Think Alike and Lecture as Exposition.

Anime & Manga

 * In the Hot Springs Episode of Neon Genesis Evangelion, they use precisely this grating dialogue when they destroy an angel with the help of thermal expansion, which by amazing coincidence, Shinji had been studying that very day.
 * Totally worth it for the line "Thermal expansion... how embarrassing", though.
 * The example used by Asuka was pretty sweet, too.
 * Episode 3 of Night Wizard featured a millenia-old puzzle about the color of a burning metal's flame guarding a Cosmic Keystone. No points for guessing what the lecture earlier on in the episode was about.
 * Done rather effectively in Full Metal Panic!! The Second Raid, where during a mission briefing one of the SRT members makes a joke about
 * An episode of Transformers Victory had an early scene of Wingraver and Dashtackler teaching Jean about levers. Sure enough, later in the episode, he came up with a plan to use a lever to free a trapped human.
 * Kakashi's lecture about revenge before the Sasuke retrieval arc of Naruto. Guess what becomes one of the central themes of the manga (and not just to Sasuke)?
 * During Naruto's Rasenshuriken training, he finally nails it after asking Kakashi how to "look towards right and left at the same time", whereupon Kakashi creates a shadow clone, inspiring Naruto to use shadow clones to do two things at once. The same phrase uttered word-for-word by Fukusaku later during his Sage Mode training gives him the idea to overcome his obstacle, again by using Shadow Clones.
 * Doesn't exactly help the protagonist solve anything, but a plot point is explained this way in The Vision of Escaflowne. Specifically, a class lecture drops off the random tidbit that . Meanwhile the main villain . Hmmmmmmmmm.
 * Gon in Hunter X Hunter escapes from a powerful enemy by remembering a lecture on how to swindle antiquarians.
 * In Pokémon Special, Pearl is told by the Fan Club President that in order to prepare for Platinum's first Super Contest performance, each member of the Sinnoh Trio must do one different part. Much later on in the story, when everyone is freaking out over the revelation that Team Galactic is planning to blow up the three lakes, Pearl notices the ribbon that Platinum's Empoleon won and remembers the President's words. He then declares that each of them must go to and protect one different lake, marking the first time in their journey together that the three of them would have to travel alone.
 * Pokémon: the actual lecture is not shown on-screen, but the only reason Ash et. al could get out safely from the S.S. Anne disaster is that Misty studied the ship's layout for a class project.
 * Your Name: Early on, a literature lesson introduces the concept of kataware doki, a kind of witching hour at twilight where reality blurs and one might encounter something inhuman. Later, Hitoha teaches Taki-in-Mitsuha about musubi, how connections can be forged across time and things taken into the body join to the soul. Both these things play vital roles in the climax.

Films -- Live Action

 * Indiana Jones and the Last Crusade. Indy mentions in class that there is no buried treasure and X "never marks the spot" Later in the film he discovers that the path to the Tablet is under the a number ten, which turns out to be the Roman numeral X, so X marks the spot.
 * The titular character of Fresh, while suffering a Heroic BSOD after his would-be girlfriend was killed in when an Ax Crazy gangster shot up the playground when he was losing at basketball, gets berated by his father who only notices how poor his son's chess game is. Still, the chess lessons on the importance of exploiting your opponents character and sacrificing any game piece if it means winning come in quite handy  while relying on the fact that he is Just a Kid to stay Beneath Suspicion.
 * Variation occurs in Monty Python and the Holy Grail: at the beginning, a castle guard gets distracted by King Arthur's "coconut horse" and goes into great length talking about whether swallows could carry coconuts into England. Toward the end of the film, the Bridgekeeper asks, "What is the air-speed velocity of an unladen swallow?" Arthur remembers a detail discussed by the original guard and asks, "What do you mean? An African or European swallow?" which causes the Bridgekeeper to fall into his own trap. In the original ending, which was cut at the last minute, the English army was saved by sparrows dropping coconuts on the French.
 * Slumdog Millionaire uses a justified version of this trope to great effect. The entire plot is centered on the protagonist explaining episodes in his life that have given him unexpectedly useful knowledge.
 * In Shorts, Helvetica does her science projects on female wasps and their habit of "male-stuffing". Later on,
 * The Devil's Backbone has a borderline example: in one scene, the boys are in class learning about prehistoric man, and Carmen shows them a picture of hunters taking down a mammoth as a group. The image is invoked at the end, when the boys
 * Elwood Blues goes on one of these lectures to his brother in The Blues Brothers on the benefits of a cop car: cop shocks, engine, suspension, etc. This comes in handy with the utter vehicular chaos that ensues.
 * Starship Troopers has Sergeant Zim demonstrate the importance of combat knives despite it being a 'nuke fight', citing, "The enemy cannot push a button, if you disable his hand!" This comes in handy later when Carmen Interestingly enough, she wasn't actually at that lecture, being a fleet pilot, and all.
 * In Eclipse, Jacob's family tells a story about how a woman helped distract a vampire by cutting herself, which Bella does in the final battle, also she does something smarter than stabbing herself in the stomach. Also, Jasper warns everyone not to let a vampire grab you under any circumstances..
 * The first Jurassic Park sees paleontologist Dr Alan Grant scare a poor child with detailed information about the velociraptor pack mentality and how they tend to eat their prey.
 * In Copycat, Sigourney Weaver's character gives a lecture on serial killers and at one point asks all the men in the audience to stand. Several of the men's faces are then projected on the screen behind her as she explains that men are more likely to be serial killers, and how they look just like everybody else. Naturally, the movie's villain (a serial killer himself) is one of the men projected on the screen.
 * Ranger LeBoeuf in True Grit drops a throwaway hint to Mattie Ross about a, just moments before.
 * King Ralph could practically be the naming of this trope. At the end, when Ralph is looking for a way out, he tells an aide that he usually ignores what the aide says, that aide once said, 'I think we made the right choice'. Ralph remembers that and asks who the other candidate is, and the denouement begins.
 * In Mean Girls Cady uses this to win the Mathletes competition. She harks back to an old maths class to answer a question about limits.
 * Timeline begins with a history lesson the the archeologists. They go back in time and need to use exactly what they just learned.
 * Black Death has the method of mercy-killing.
 * Jett Jackson: The Movie, the made-for-TV movie of The Famous Jett Jackson: After Jett and his TV character Silverstone have switched places in each other's universes and have been living in each other's shoes for a while, Jett's grandmother later approaches Silverstone in a quiet moment, where it's revealed that she'd already figured out he wasn't her grandson. When Silverstone asks how she knew, she explains that when Jett was born, she looked him in the eyes and knew that from thenceforth she'd always be able to know and identify him..
 * Let's not forget Pirates of the Caribbean. In the first movie, Will frees Jack from prison using a bank as a lever on the prison door. In the third movie, when Jack is captured by Davie Jones, guess what he does to the prison door?

Literature
"Little did Langdon know this information would save his life in a matter of hours."
 * Hermione constantly saves the day in Harry Potter because she was the only one paying attention in class when the relevant magic was discussed.
 * This sort of thing is even done across books. In the fourth book, Dumbledore casually mentions a room filled with chamber pots that he found when he desperately needed a bathroom, and then was unable to ever find again. In book 5, the room is formally introduced as the Room of Requirement, and ends up being an integral part of the story in every remaining book in the series.
 * And in the very first book, Snape quizzes Harry, and one of the questions is about bezoars. In book six, it's used to save Ron's life, although Harry re-learns about it from Snape in a roundabout way.
 * "Has no-one read "Hogwarts A History"?
 * In Dan Brown's Angels & Demons, an early discussion of how much a small flat object can slow down a falling person saves Robert Langdon's life when he uses a window cover to slow his fall from an exploded helicopter. This is also lampshaded at the time the discussion takes place.

"Arthur: You know, it's times like this I wish I'd listened to my mother. Ford: Why, what did she say? Arthur: I don't know, I wasn't listening!"
 * Modern pulp author Matthew Reilly uses this to a ridiculous degree. Two examples:
 * In Temple, there's a throwaway sentence from the protagonist about how he'll need to change his PIN number after reading a story in the paper about how most people use their birth dates as pass codes. Guess how he defuses the superweapon his brother worked on?
 * Guess again, it wasn't his birthday. But that example was used as a starting point.
 * In Area 7, a precocious youth found in the middle of a government base delivers a buttload of the kind of trivia kids that age accumulate and share at any opportunity, including how komodo dragons are sensitive to changes in the Earth's magnetic field. So, of course, there's a scene where the main character has to fight off komodo dragons in a watery pit with his magnetic grappling hook.
 * Subverted and lampshaded in The Hitchhiker's Guide to the Galaxy.

Live Action TV

 * Even Buffy the Vampire Slayer isn't above using this trope at least once. In order to defeat the evil science teacher who is actually a giant praying mantis, Buffy uses the recorded sound of bat sonar to "make [her] nervous system go kerplooey". She of course learnt about that in science class earlier in the episode (though, thankfully, from the previous science teacher. No villain should be stupid enough to teach a class their own weaknesses).
 * Of course, that's not how she ultimately kills the praying mantis. She does that with a big machete.
 * Of course. It's Buffy, after all.
 * Doctor Who used to do this on several occasions, partly due to originally being conceived of as an edutainment program. For example, in the early story "Marco Polo" the condensation of water was a key plot point, while in "Planet of Giants", that pressurised cans explode when heated was another point. The Daleks were originally defeated due to elementary knowledge of electrical conductivity. The same technique was used to teach history in its early historical stories. Though most of the explanations happened in the course of events, not given out only to later be recalled.
 * Pretty much every episode of Black Hole High (a.k.a. Strange Days at Blake Holsey High) featured this. Apparently, the unpredictable wormhole at least had the good manners to follow the state provincial-mandated science syllabus exactly.
 * Given the way physics works at Blake Holsey (namely, that its laws will bend to teach you an Important Moral Lesson), it is entirely possible that the wormhole was doing it "on purpose", and the physics lectures or experiments in act 1 were really shaping the physics weirdness in act 2.
 * Or, of course, the time traveller could have done his history research and made sure the syllabus matched.
 * A lovely children's education show called Storylords entirely revolved around this. Somehow, Mrs Framish the reading teacher, always had either covered the necessary reading skill that day, or taught it the next day, in plenty of time for our hero to use it to defeat Thorzuul.
 * Subverted in Stargate Atlantis, when Sheppard finds himself in an F-302 latched onto a Wraith Hiveship in hyperspace. He flashes back to a memory of McKay and Zelenka arguing about whether a non-hyperspace capable ship could detach from another one while in hyperspace without being destroyed, which is exactly what Sheppard needs to know. Then they ask Sheppard what he thinks, but he's not paying attention because he's flirting with the woman at the next table. He ends up not taking the risk, and has to wait for the Hiveship to leave hyperspace.
 * Crossed with I Know You Know I Know in the 'Fingers and Fumbs' episode of QI, where host Stephen offers the contestants an opportunity to go double or nothing on a forfeit by playing Rock-Paper-Scissors with him. He mentions that, psychologically, people tend to pick scissors first, because it's commonly believed that others would play rock first, and so would play paper to counter it. Phil and Dara both tie with him on scissors the first three times it happens (Phil having played twice). The fourth time, however,
 * "Hermaphrodite Nazi Sympathizers."
 * The third episode of BBC's 2010 Sherlock had Holmes fighting an assassin in a planetarium while an astronomy lecture played in the background. Wouldn't ya know, the lecture contained a clue that helped Holmes identify a painting as a fake and solve Moriarty's fourth challenge.
 * Exploited by the crew on Leverage. While setting up a con on a college student, Nate plays a professor who antagonizes the mark so that Hardison can make friends with him. The lecture Nate gives is about the prisoner's dilemma problem in game theory, and he tells the class that it's always better for the prisoners to turn on each other. Later, they put the mark in a situation where he has the option of turning on his confederates, and he flashes back to Nate's lecture and decides to do it.

Video Games
"Shirou: "It wasn't his usual harassment. Those words held an importance that I need to understand right now. -- No, saying that... Weren't all of his words a warning that I shouldn't have ignored?""
 * The Persona series:
 * Sort of used in Persona 3:
 * Used in a similar manner in Persona 4,
 * Kingdom Hearts 358 Days Over 2 has a kernel of wisdom, courtesy of Xaldin—he warns that the bridge to the Beast's Castle is the only point of access, so if a powerful enemy were to attack the area it'd come from there. Both foreshadows the appearance of such a boss later in the same game, and is a Call Forward to Kingdom Hearts II where Xaldin engages you on the bridge.
 * In Birth By Sleep Eraqus tells Aqua classified information only Keyblade Masters are allowed to know, but this knowledge isn't revealed to the player.
 * It gets glossed over early on in Soul Reaver that vampires are vulnerable to certain sound frequencies, but this doesn't serve much purpose except for a sound-based attack spell and a non-canonical deleted ending. Then three games later Defiance pits us against, a vampire with Super Senses who can only be harmed by ringing a series of giant gongs.
 * Present in the Fate route of Fate/stay night, as Archer decides to give a few hints about his magic to Shirou, with plenty of sarcasm and veiled threats thrown in. Our hero later uses all of the information gained.


 * In the first year of Grim Fandango, the janitor demon lectures you that spraying the fire extinguisher on the packing foam causes an explosion. You use this information later on in the fourth year, where.
 * Used in Half-Life 2 Episode 2: At White Forest, a rebel is teaching others about the effectiveness of an AR 2 Combine ball against hunters, a while before.
 * In Space Quest V you begin the game by passing a test that gives ridiculous answers to questions like "how to best defeat an android bounty hunter" -- . Several of those turn out to be answers to in-game puzzles.
 * mention the obvious one in BioShock (series)?

Web Original

 * In the Whateley Universe story "Ayla and the Tests", Phase misses several days of school and gets a big lecture in the lab section for Powers Theory class from Dr. Yablonski on Warper powers and warp displacement fields of 'giants' (Warpers who use their power to apparently grow to huge size). In a Lampshade Hanging, Phase even refers to him as Mister Exposition. Guess what Phase faces when they go to Boston? A forty-foot giant who's a warper.
 * In "Ayla and the Birthday Brawl", Ayla shows (s)he knows too much about taking out mutants with powers, and she talks all about it on the way back from martial arts class. What she says about fighting Package Deal Psychics saves Lancer's life in the big final fight.
 * The students at the Hyperion Academy used the information they learned in about electro-magnetism during their weekly physics class to defeat the villainous Lodestone.
 * Lampshaded in A Very Potter Musical: at the beginning, Snape asks if anyone knows what a Portkey is. His next question is if anyone knows what Foreshadowing is.

Western Animation
"Max: How come we never have a mission to some place I'm studying? Like the Yukon. Rachel: Now wouldn't that be convenient."
 * Generally averted in Code Lyoko. Though some classes at the start of an episode may have a coincidental link to future events (for example, a lecture about nuclear energy production happening just before XANA attacks a nuclear power plant...), the specific content of the lecture would hardly be of any help for the heroes, since Jérémie and Aelita know probably more than the teachers already. At best it will be mentioned as a joking aside. If the class directly influences the events, it's usually because it gives Jérémie an idea that he next tries out—like the biology class about ants that prompts him to create the "Marabounta".
 * Parodied in the American Dad episode "Stan's Night Out". Stan watches a television show about gardening, where the host says that you can start a lawnmover with the first pull, if you stand on the back wheels. Later, when he's trapped by a ruthless crime lord in a shed, he sees a lawnmover, and proposes a wager; if he can start it ten times in a row, the bad guy will let him go. So, he stands on the wheels, pulls the rope... and the lawnmover doesn't start.
 * In what has to be a spoof, an episode of Kim Possible has Ron tutoring Kim in Home Economics. Later on, Kim uses Ron's advice when Shego has her trapped in a giant industrial mixer. While facing certain death from the large mixer blades, Kim Possible grabs the mixers, stays/spins with it, follows it up into the air, lets go of it and defeats Shego by doing a jump kick that screws with the laws of physics. All thanks to a lesson about cooking where Ron says to her how she shouldn't fear the mixer but instead be one with it.
 * In a later episode, Executive Meddling forced them to have a lecture about good nutrition and the five food groups. Ron falls into a vat of the Titan Formula and becomes a junk-food-eating-monster. Luckily Kim and her brothers know the cure for that!
 * And to drive home the fact that they had to make this episode, Ron gives a And Knowing Is Half the Battle by warning kids not to fall into vats of experimental super growth formula.
 * In The Magic School Bus, usually the dangerous situation is the lecture, or an extension of it, and the resolution always conveniently requires them to recap everything they've learned that day. Arguably justified as intentional on Ms. Frizzle's part as an aspect of her teaching style.
 * Normally subverted in Mighty Max whenever Max comes up with a clever plan to save the day. It's usually not until after the villain is defeated that Max gives the Chekhov's Lecture, either to the other characters or directly to the audience in the Edutainment sections at the end of each episode.
 * Referenced on The Simpsons when a tipped salt silo from the local cracker factory melts all the snow surrounding the school and Martin tastes it and declares that it melted with "a little help from our friend Sodium Chloride!" He then gets punched in the gut by Nelson.
 * Another episode has Bart rush out of a class on Roman numerals to steal the town's lemon tree back from Shelbyville. Later on in the same episode he ends up in a room with a choice of doors labelled I-X and a piece of paper that claims that the seventh door is the exit and all the others contain man eating tigers. In a subversion, he doesn't even remember the class but makes the right choice anyway thanks to his knowledge of Rocky films.
 * And in case you're wondering, the film in question Bart comes up with?
 * The Phantom of Flying Rhino Junior High frequently transforms the school into a death trap based on whatever lecture (or sometimes just topic of discussion in general) the children were having that day. This naturally leads to the students who were paying attention being the ones to get everyone out of trouble.
 * Generally averted in Winx Club: The only time anything we've seen them learn in class has been useful to the plot was when Tecna fired a spell she learned in class at Professor Avalon. One can debate that even that spell didn't work properly.
 * Total Drama World Tour has Sierra mentioning that Cody's birthday is April 1 during the first episode. In Aww, Drumheller,
 * Parodied in Megas XLR, when Coop has sudden flashback to his high school science teacher scholding him for not paying attention and saying that one day he might need her lesson. She was right, as what she was talking about happens to be contain informations about bad guy's of the week main super weapon and Coop really doesn't remember them.
 * In one episode of The Mummy animated series, Alex manages to avoid being crushed by falling ruins, commenting on his recent geometry lesson really came in handy.
 * Inverted and parodied in Max Steel:
 * Inversion: Josh is required to write an essay on the Cold War with a fresh angle. A mission he goes on later that day requires him to go through escape tunnels in Washington D.C. (something vaguely mentioned in the lecture), and he uses that knowledge to get an A+ on his essay.
 * Parody:


 * In the Avatar: The Last Airbender episode "Bitter Work," Iroh teaches Zuko all about the principles of lightning bending. These same principles are later applied when
 * Subverted in SpongeBob SquarePants: Spongebob's boating class goes on a field trip to a famous boat, where Spongebob annoys everyone (Mrs. Puff in particular) by rattling off useless trivia. The boat then appears to have activated and is barreling towards town, so Spongebob is called on to save the town.