How Did You Know? I Didn't.

"Bart: Dad, you killed zombie Flanders! Homer: He was a zombie?"

- The Simpsons, Dial Z For Zombie

Stock Phrase uttered in which our hero attacks a seemingly Innocent Bystander but discovers that the bystander was not what he appeared.

Goes as follows:

"Sidekick: How did you know he/she was an evil X? Hero: I didn't..."

Let that sink in awhile.

Borders on Indy Ploy for charging in with no other plan and Moral Dissonance when you realized that our hero had no problem killing someone and just happened to land on the accepted side of What Measure Is a Non-Human?. Alternatively, sometimes the hero might have known the truth all along and just be yanking the chain of the person asking the question.

However it can also be used to show the hero's courage, as in: "How did you know that bomb wasn't real?" "I didn't." (It's also used as a punchline, where the hero retroactively freaks out when they realise how much danger they were in.)

Not to be confused with You Just Told Me, although the same phrase can be used for that purpose too. Also not to be confused with Accidental Truth. See also I Have No Idea What I'm Doing.

Anime and Manga
"Al: Oh wow, Ed, how did you know that wasn't the Colonel? Ed: ... it wasn't?"
 * An extreme example occurred in Saiyuki when the entire group sans Gojyo decided that the next time they saw Gojyo, they'd beat the crap out of him for leaving their group and throwing off the dynamic. They meet an evil doppelganger Gojyo and naturally start to beat the crap out of it. When they discover it isn't Gojyo they decided to pretend it is and proceed to tear it to shreds as a catharsis.
 * Especially funny because the guy disguised as Gojyo fully expected them to pull the "How can I bring myself to fight my friend?" act... Ehehe.
 * This speech follows pretty much every victory of Irresponsible Captain Tylor. It comes to the point where his second in command begins to anticipate the 'I didn't' reply. He becomes Genre Savvy enough that he decides to imitate Tylors style when mounting his rescue operation. And yes, it also works when he does it.
 * On Yu Yu Hakusho Yusuke is faced with a Spot the Imposter predicament and is told he has to punch the culprit as hard as he can to expose him. He correctly chooses Kuwabara but only did so because he knows Kuwabara is used to being hit by him while ruling out the others. He did mention before doing so that he hoped he was right.
 * In an OVA from the first series of Fullmetal Alchemist, Ed and Al get caught up with the Homunculi after Envy shapeshifts pretend to be a captured Winry. They escape, chaos ensues, and Edward ends up taking out Envy who's disguised as Roy Mustang.

Comic Books
"Black Canary: "Hey, how'd you know about Cheetah's healing factor?" Huntress: "Healing what now?""
 * The Avengers #1½: The Hulk is attacking a Dr. Doom robot and is asked (after he smashes it) how he knew it's a robot. "Robot?"
 * Birds of Prey #97: Cheetah tackles Black Canary and threatens to rip her throat out. Huntress shoots Cheetah in the shoulder, face and throat.

"Slate: Pretty impressive, by the way. Timing Boxer's falling into that truck so he wouldn't be hurt. Lee: Beat Panel. What truck?"
 * In Marvel's G.I. Joe comic, Sgt. Slaughter is confronted by two Gung-Hos. He punches one in the face who turns out to be the shapeshifter Zartan. When asked how he knew which one to punch, Slaughter replies, "Lucky guess."
 * A variant occurred in the comic book Spy Boy.
 * An issue of Fantastic Four had a future version of Namor and Black Panther coming to the present for some reason or another. It's revealed that Reed's devotion to his work would eventually drive his family away from him, culminating in Susan running off with Namor (as she's been on the verge of doing before). Reed takes this news calmly, then shoots Namor in the head. In this case, he suspected, but did not know for certain, that this was a plot orchestrated by Doom, and that the future Namor and Black Panther were robot impostors.
 * Reed based his decision on 'well I can't believe Doom solved that external power supply problem for that type of force field so Namor and Black Panther must be androids holding the power suppplies'. He was so certain Doom couldn't have solved a problem he couldn't even from decades into the future that he was willing to risk killing what was possibly the real Namor to prove himself right.
 * The Maxx: the Big Bad takes what is assumed to be a girl hostage. Maxx smashes the girl's face into a car, leaving the villain to wander off in a fit of anger. It was a CPR dummy, but Maxx very poorly tries to cover up his surprise it wasn't a real person.
 * Runaways Volume 3 #10, Chase tells Victor to swallow the mutant cobra eggs in order to destroy the mother. Chase admits that he didn't know that it would work but figured worse case scenario they could rebuild Victor.
 * A more serious example occurs in the first volume: after a vampire had taken out nearly the whole team, Karolina offered to let him bite her in exchange for letting Molly live, claiming she wanted to die. He bit her—and spontaneously combusted, since she's a solar-powered alien. When Alex asked her how she knew that would happen, she admitted that she had actually been trying to commit suicide by proxy.
 * The Umbrella Academy: Dallas #1 features the Lincoln memorial statue come to life and terrorize the District of Columbia. To stop it Rumor says "Mr. President. I heard a rumor you were assassinated." This conjures up a statue of John Wilkes Booth who shoots the rampaging Lincoln. Hargreeves arrives on the scene and ask how she knew that would work. She replied "I didn't."
 * From Fallen Angel: This exchange after Lee drops Shadow Boxer off a roof and he lands in a hay truck.

"Mystique: How'd you know I'd be able to untie myself before the shooting started? Wolverine: I didn't."
 * Wolverine Vol 3 #62, Wolverine and Mystique are tied up in front a Mexican firing squad. They escape.

"Victor: So... They told you about my healin' factor. Dum-Dum: He heals?"
 * In The New Avengers, Dum-Dum Dugan shoots Victor Creed in the head:

"Giant-Man: Careful! Her healing factor isn't as strong as Wolverine's! Mettle: You knew she'd survive that, right? Hazmat: Yeah. Woulda done it anyway."
 * In issue #1 of the original The New Warriors, Night Thrasher drops Richard Rider off a rooftop in order to re-activate his Nova powers (which Richard thought he'd lost after returning to Earth... long story). You can guess what Nova's next question and Night Thrasher's response are.
 * In Avengers Academy #25, Hazmat blasts X-23 with a toxic blast after X-27 stabs Mettle:

Film
"J: "You're back." K: "No." J: "How'd you know his head grows back?" K: "It grows back?""
 * Men in Black II: Amnesiac Agent K shoots alien pawn-shop clerk Jeebs in the head. Again.

"Piper: "How did you know I was wearing a bulletproof vest?!" MacGruber: "You were wearing a bulletproof vest? Awesome!""
 * The Naked Gun 2 1/2: Frank Drebin is being "honored for his 1000th drug-dealer killed." He responds: "Thank you. But, in all honesty, the last two I backed over with my car. Luckily, they turned out to be drug-dealers."
 * Get Smart: Max is asked  The chief asks how Max knew. The answer: Lucky guess.
 * Papillon (1973) takes shelter in a leper colony after his escape, and is invited to share a cigar by their leader. Despite his fear of catching the disease, Papillon does so. There's a moment of silence from the leader (who clearly expected him to refuse) before he asks how Papillon knew he had dry leprosy, which isn't contagious. When Papillon replies that he didn't know, it breaks the ice between the two men.
 * In the movie version (but not book) of The Hunt for Red October, Jack Ryan and Captain Mancuso invoke this trope, in regards to which way Captain Ramius would turn for the next "Crazy Ivan" maneuver.
 * MacGruber:

"Gloria: Aren't we forgetting the small matter of dead bodies? Rosemary: How did you know about the other bodies? Gloria: What other bodies? Rosemary: *relieved* Oh, you didn't know about the other bodies."
 * An interesting spin on this trope is that it was the final straw for Piper and he temporarily quits the team.
 * In The Day of the Jackal, Commissioner LeBel believes that a member of the French Cabinet is leaking confidential information to The Jackal. LeBel plays a tape of a telephone conversation to the entire Cabinet, showing that one of their members had been betraying them. Another Cabinet minister asks him how he knew which one of their phones to tap. "I didn't so I tapped all of them." Only the Prime Minister is unsurprised, since he gave LeBel unlimited authority in the first place.
 * The question is reversed in Splice when
 * Keeping Mum, after Gloria and her daughter Holly learn in short order that the new housekeeper is a paroled murder who killed again, and is also Gloria's long-lost mother.

"Mike: (imitating the bank manager with a rifle) What ?! He was robbing my bank! I thought he was just a clown!"
 * Diamonds Are Forever has an example of one of the mooks doing this. The mook throws Plenty O'Toole, one of the Bond girls, off of a high floor, and she lands unharmed in a swimming pool below. When Bond compliments him on his aim, the mook replies, "I didn't know there was a pool down there."
 * Zigzagged in Pirates of the Caribbean: On Stranger Tides. Blackbeard forces Jack to play a variant of Russian Roulette with four unloaded pistols and two loaded ones, and Angelica (his daughter, Jack's love interest) as the target. When Jack takes the option to jump instead, Angelica asks her father if he knew which guns were loaded. He says he did, but considering his later actions there's no reason to believe he would care about putting his daughter in danger.
 * Used on the Riff Trax for the opening of The Dark Knight.


 * The Conspirator: Aiken mistakingly asks a question at trail without knowing the answer. It was dumb luck that it worked in his favor, and he confesses this to his friends.

Literature
"Simon: How'd you know he wasn't serious about it? Jeremy: About what? Simon: About . How'd you know he didn't mean it? [after a short pause] Simon: You didn't know, did you. Jeremy: [...] I suppose you could say that I didn't know for certain--but I hoped."
 * In High Fidelity, after spying on Jeremy, Simon asks Jeremy how Jeremy had been sure that had been bluffing about.

Live Action TV

 * Ned's Declassified School Survival Guide has Cookie as the "new kid" posing as a British person to seem more interesting to his new class. Turns out the class already has a British kid, who calls him out on it. Cue Cookie stammering and then trying to save face by sputtering miscellaneous stereotypical and nonsensical Britishisms. Then it turns out he was faking as well; he's from Ohio, not Britain. Cookie asks "Then how did you know I wasn't British?". The response? "I didn't. I just took a stab and you caved.
 * The Made for TV Movie Nick Fury, Agent of SHIELD (with David Hasselhoff as Nick Fury). Nick shoots Director Princer, who turns out to be a Evil Life Model Decoy, but Nick didn't know this. He just hated Princer.
 * In the Buffy the Vampire Slayer tie-in comic "Ring of Fire", Kendra attacks a man who turns out to be a demon. Xander asks how she knew. She didn't.
 * A Bad Boss example is in "Band Candy". Mr Trick accuses a worker of sampling the cursed product and Neck Snaps him. Ethan Rayne asks Trick how he knew the worker was stealing. Trick replies that he didn't "And now no-one else will either."
 * In an episode of Angel, Wesley shoots somebody who turns out to be a robot double, to dramatic rather than comedic effect. The point in that case was not that Wesley makes a habit of casually killing people, but that in this one instance he was pushed over a line he wouldn't normally cross.
 * Knowing how he treated him as a child, the shooting is 100% justified. However, Wesley does shoot one other person in S5, in the leg, in anger. It seems that getting Wes angry when he has a gun nearby is a VERY bad idea.
 * There was a variation in Spooks once (season 2 episode "I Spy Apocalypse"), where the guy the hero shot was real (and human), but the gun turns out to have been rigged.
 * Deep Space Nine. When Worf realises General Martok is suffering from post-traumatic stress disorder as a result of his imprisonment, he challenges Martok's leadership in the Klingon tradition, then lets him win the subsequent knife fight. Reinvigorated, Martok rallies to his command and saves the day. Later, knowing that Worf lowered his guard deliberately, Martok asks how Worf knew he wouldn't kill him. Worf replies simply, "I didn't". The general is so impressed he officially adopts Worf as a member of his House.
 * In the White Collar season one finale, Peter shoots  in the chest to stop him from shooting  . When   asks how he knew   was wearing a vest, he responds with this.
 * The Moral Dissonance is averted, however; the "victim" in this case?.
 * Variant in the Doctor Who episode "Amy's Choice".
 * A variant in Bugs happens when the team's weapons expert is wired with a bomb to force him to act as a sniper to remove someone for the Big Bad. While the tech specialist is trying to disarm it the Big Bad realises he's not going to make the shot and moves to kill the target himself. The weapons expert pulls away from the tech and kills the Big Bad, then the tech removes a chip from the bomb and it deactivates. Cue the tech being lambasted for taking so long and replying that she just removed anything, choosing probably being blown to bits over certainly being.
 * In one episode of season nine of Smallville, Oliver Queen is standing on a podium which has been rigged to blow when he steps off. Clark arrives towards the end of the episode, having learned that the bomb was  only to find that  . How did he know? He didn't, and instead waited until the room was clear so that he could.
 * Partially subverted in Farscape, where in an Enemy Mine situation where they're all hostages, Rygel arranges for Scorpius to be shot by one of the criminals. Then, just as he's about to be dragged off to the getaway vehicle, Rygel declares he wants a souvenir, and approaches the "corpse." Once he's sure nobody's listening, he apologises... and Scorpius asks him how he knew he was wearing body armor; Rygel answers that he only suspected it, but didn't actually care that much if he was right or wrong.
 * ''Kaizoku Sentai Gokaiger", "The Amazing Silver Man" The crew accidentally stumbles upon an Action Commander planting a bomb to blow up the city and demands to know how they knew his plans. They didn't, he just told them.

Mythology

 * This troper once read a story about a samurai who nobody would hire because his severe ill luck causes his side to always lose. In the end, when defending an underwater city from an evil dragon, the samurai spits three times on his third, and last, arrow for good luck, then shoots the rampaging dragon between the eyes, after which it promptly falls down stone dead. The king of the underwater city asks "How did you know human saliva is deadly poison to dragons and demons?" The hero immediately responds, "I didn't"

Newspaper Comics

 * Alice in Dilbert deduces that a coworker must either be an unhelpful moron or a robot send to destroy the company from within, and proceeds to tear his (robotic) head off. Dilbert asks how she knew it was the robot-option...

Video Games

 * The player gets an opportunity to have his own How Did You Know? moment in the (chronological) beginning of Max Payne 2. No game mechanic prevents you from killing an innocent bystander at a crime site, a cleaner. In fact, you will be rewarded for doing so by him not leading you into an ambush.
 * Max himself gets one when he Only later does he actually find any evidence that  Max, ever the melodramatic sort, states that thinking for even a moment that the future revelation justified his prior actions damns him just that much more.
 * A similar example in Deus Ex. If you kill a  on sight, the guy will later turn out to be a spy, and   will even praise you for being just the right type of trigger-happy paranoiac they need.
 * Assassin's Creed had Al Mualim and Altair using almost these exact words during one of their discussions. Al Mualim asking and Altair answering.
 * The popular gamemode for the Half Life 2 mod Garry's Mod titled "Trouble in Terrorist Town" revolves around trust/mistrust in a town full of terrorists, 1 in 5 of which are united traitors, seeking to kill the innocent terrorists, similar to the Mafia game people play on forums/IRC/at social gatherings. Occasionally someone gets shot for no real reason, or are accidentally pushed off a high ledge or get just plain unlucky and die suddenly when they're a traitor. The result is an upset traitor, and a confused, but happy group of innocent terrorists who have one less threat to deal with. Especially funny when playing with friends who you actually know, rather than random players.

Web Comics
"Mom: You've brought boys over, Susan? Susan: No, Mother, just this one. The other two are girls; they're checking out our TV. Justin: Hi. Mom: I'm not comfortable with him spending the night here, Susan. Susan: Don't worry, Mother. He's ... gay. He's gay, so he won't try anything with me. Mom: Ugh... men who like men... pure insanity. Fine, he can stay. Maybe his homosexuality will rub off on you, and you'll find yourself a decent woman. I'll leave the lot of you alone now... I'll be upstairs. Susan (thinking): Hehe... she bought it... Justin: So... how'd you know that I'm gay?"
 * Set up here in El Goonish Shive (the full exchange doesn't come until the next strip):


 * (Dis)honorable mention to Vaarsuvius from Order of the Stick, who killed Daimyo Kubota without an afterthought due to some Genre Savvy assumptions without actually knowing petty details like Kubota's crimes or even his name.
 * A comical version done here.
 * Night Owls #134, Ernie asks Bill how he knew Louie's magical gaspipe was inert. Bill doesn't know what he's talking about.
 * This Cyanide & Happiness comic.

Western Animation
"Bart: Dad! You killed the zombie Flanders! Homer: He was a zombie?"
 * In a Xiaolin Showdown episode where Kimiko is fighting an evil robot duplicate, Omi jumps high in the air and takes one of the Kimikos out in one swipe. The defeated robot explodes, but, when asked about how he knew which one was which, Omi admits to having guessed. Not quite as extreme as the page description otherwise implies, since if he'd kicked the real Kimiko, she'd have probably just rubbed her kicked spot and been annoyed... not actually exploded.
 * This really surprised Clay and Rai though because up until that point, Omi knew ahead of time every trick thrown at him throughout the episode, including Rai constant attempts through Shen-Gong-Wu battles and card dealings. When he said that he "guessed" that time, they were aghast he didn't/couldn't use his Tiger Instinct.
 * The Simpsons Halloween episode "Dial 'Z' for Zombie", Homer shoots the zombie Flanders.

"Grampa: (holding a hammer and stake) "Quick! We must kill the boy!" Marge: "How do you know he's a vampire?" Grampa: "He's a vampire!?" (runs off screaming)"
 * Another example from another Halloween episode:

"Leela: "Fry, you did it!" Fry: [reenters, zipping up pants] "Did what?""
 * Futurama "The Deep South", the ship is sinking and taking on water. Fry rushes offscreen, we hear a toilet flush, and the water starts to drain away.

"Commander: "How did they know exactly where to hit us?!""
 * Megas XLR: "The Driver's Seat" Coop accidentally teleports his Mega-Slush into a critical component of the Glorft mothership.

"Recruit: "How did you know that they would retreat before we rammed them?" Commander: "I didn't.""
 * Robot Chicken: In the Yellow Submarine meets The Hunt for Red October sketch, Ringo didn't know the hole would stop the missile from hitting the Blue October; he was still on acid and said whatever came to him.
 * The Wing Commander Academy cartoon had an example where, faced with a Kilrathi carrier, the commander decides to play chicken with it. This works, and the carrier retreats.

"Roger: How did you know I was fireproof? Even I didn't know that! Wait, did you know? I'm going to assume "yes" to preserve the friendship."
 * In American Dad

"Stan: So how did you know I was wearing a vest? Woman: I didn't, but I have a rowboat, so it's easy for me to dump stuff. Did you know about mine? Stan: No, I was composing your suicide note when you stood back up!"
 * Again in "When a Stan Loves a Woman." At the shooting range, he and a mysterious woman simultaneously shoot each other. Cut to a restaurant:

"Woman 1: That was amazing! Woman 2: You saved her life! Woman 3: Thank God you know CPR! Quagmire (confused look): What the hell is CPR?"
 * In Part Two of the pilot for My Little Pony Friendship Is Magic, the main characters are attacked by a manticore. They fight back until Fluttershy calls for them to stop, much to their surprise. She approaches the monster, nuzzles it, and the manticore reveals the source of its fury: a thorn in its paw. After she removes the thorn, the manticore lets them pass. Twilight Sparkle asks how Fluttershy knew about the thorn, and the pegasus replies that she didn't, but assumed the manticore just needed to be shown kindness.
 * This exchange from Family Guy after Glenn Quagmire saves a woman who is having a heart attack while changing at the mall:

"Mr. Blake: But how did you know she would step in the middle of the boat? Fred: I didn't. That's why I set up traps all over this place!"
 * Also the basic frame of this trope is present in a joke in the (same) episode where Peter is blind. After Peter saves Horace from a burning building he is asked how he found the courage to risk his life inside said burning building, to which Peter responds "That freaking place was on fire?!"
 * Scooby-Doo! Mystery Incorporated "The Dragon's Heart"

"Archer: (calls Ray on his cellphone, then snaps a picture of a bomb in his mother's work desk) I need you to help me... Ray: (instantly) Blue and yellow. Archer: You mind looking at that for more than a half-second? Ray: I wired the damn thing. Ass. Archer: Well, I didn't know that. Ass. (clips wires) Ray: Holy shit, you actually did it? I was lying."
 * In Archer:

Real Life

 * In the Soviet Union in 1983, a Soviet early warning nuclear missile detection system gave a false alarm that a US nuclear missile was headed for the Soviet Union. The officer in charge at the time decided it was a false alarm and chose not to report it to his superiors, deviating from standard procedure and possibly averting World War III. Of course it was a false alarm caused by a mechanical error, but how did he know?