->'''Reggie:''' Take this walkie talkie and hide. You'll know what to do.
->'''Twister:''' I will?
-->-- ''WesternAnimation/RocketPower''

Stock phrase when the hero is presented with a PlotCoupon.

Played for laughs when the item in question [[ItMayHelpYouOnYourQuest May Help You On Your Quest]].

Usually translates as "We're not going to tell you what the magic amulet does yet in order to keep the audience in suspense," or, more cynically, "We don't actually want to do the work to prevent a DeusExMachina ending, so we're throwing this in now to make it look like we had it planned all along." Sometimes, in a very long arc, it can also mean "We haven't worked the resolution out either, so we're tossing in a random item now. The author will work out what it's for later." Perhaps they know that an [[UnspokenPlanGuarantee unspoken plan always works]]?

The general gist of things is that the item will go forgotten for the rest of the story, and then, at a key moment, when all else has failed, he'll suddenly realize what the PlotCoupon is for, pull it out, and save the day.

Can refer to a character's [[SomedayThisWillComeInHandy generally-useless special ability]] instead of an actual physical object.

Often parodied these days by having the coupon itself be something not merely innocuous, but outright ridiculous.

An offshoot of ChekhovsGun and often used as ammo for SchrodingersGun. Related to FigureItOutYourself, and occasionally answered with IJustKnew.

Expect things to go badly if the character ''[[SubvertedTrope doesn't]]'' [[SubvertedTrope know what to do with it]].
----
!!Examples:

[[foldercontrol]]

[[folder:Anime and Manga]]
* In ''RaveMaster'', Haru Glory, wielder of the Ten Commandments sword, initially can only create the "Explosion Sword." Eventually, he is given the Rave of Wisdom, which gives him knowledge which is only gradually unlocked and told "YouWillKnowWhatToDo." From then on, he's able to [[NewPowersAsThePlotDemands whip out new forms of his sword as the plot demands.]] This comes with understanding of ''how'' the powers work, so he can from then on also call them forth at will.
* This was sort of the case in ''Anime/YuGiOh'' where Kaiba gave Yugi his copy of Fiend's Sanctuary before the FinalBattle with Marik in Battle City. Kaiba knew it was an effective weapon against the Winged Dragon of Ra, but he just told Yugi to take it and nothing else, figuring that if Yugi was capable of winning at all, he could figure out how to use it on his own. Fortunately, he did. (It's absent from the dub, but Kaiba calculated when he gave Yugi the Fiend's Sanctuary card that it would do nothing more than increase Yugi's chances against Marik from 3% to 20%; he still doubted Yugi stood much of a chance. Kaiba told Ishizu that he only helped Yugi to prove her wrong about the bond between then when he lost. At least that's what he claimed. And as stated, Yugi ''did'' win, the card helping a great deal.)
* Zigzagged in the first season of ''Anime/YuGiOhGX''. After Judai defeats Daitokuji-sensei, he gives Judai the [[TomeOfEldritchLore Emerald Tablet]] before his physical body dies. Later, before he duels [[BigBad Kagemaru]], Judai finds a card called Sabatiel - The Philosopher's Stone in the book, and figures he should add it to his deck. Just as he's at the end of his rope, he draws the card, but still has no idea how to use it. Daitokuji's spirit appears, and is about to tell him ''exactly'' how to use it, but then he's [[ItMakesSenseInContext swallowed by his cat Pharaoh]] before he can tell Judai the complete effect. Fortunately, Judai is able to figure that out on his own.
[[/folder]]

[[folder:Comicbooks]]
* Comic book example: In ''Comicbook/{{Exiles}}'', when Beak joins the team, they're told that he has "an important task ahead of him" and that "several worlds will hang in the balance", essentially making the character into the PlotCoupon.
[[/folder]]

[[folder:Fairy Tales]]
* In ''Literature/CatherineAndHerFate'', Catherine appeals to her Fate, to soften her hardships. Her Fate responds by giving her -- a skein of silk thread. Catherine nearly throws it out in annoyance.
[[/folder]]

[[folder:Film]]
* The main character of the movie ''{{Paycheck}}'' gets ''twenty'' of these in a NoteToSelf, but this is more {{justified|Trope}} than most examples of this trope, as well as more skillfully executed. (It's pulled off with even ''more'' flair in the original short story.)
* In the second ''PiratesOfTheCaribbean'' film: "I gotta [[ItMayHelpYouOnYourQuest Jar o' Diiirt]]!" Subverted somewhat in that it [[spoiler:really does turn out to be useless. The useful stuff was taken out beforehand.]]
* The Autobox Matrix of Ass-Kicking in ''TransformersTheMovie''. Nicely handled when, halfway through the movie, someone tries to use it and can't - it's not ''bad enough'' for the heroes yet.
* In ''Film/GalaxyQuest'', the protagonists have to admit they don't know how the Omega-13 device works, because the TV series ended on a CliffHanger before it was actually used. Fortunately, one of the [[{{Fanon}} fan theories]] proves correct.
* Parodied in ''Film/ReignOfFire'' :
-->- I'm going out. Anything happens, you know what to do.
-->- Er... no.
-->- Yeah, me neither.
* Done rather poorly in ''Film/{{Troll 2}}'', when Grandpa Seth hands Joshua a [[ItMayHelpYouOnYourQuest double-decker baloney sandwich]], saying "take this, and only use it when you really need it". The last 5 minutes of the film is not a good time to introduce a vital ChekhovsGun.
* In the move ''Film/LittleNicky'', a group of angels hand Nicky a white orb before his confrontation with his brother, relaying a message from the God that when the times comes, [[YouWillKnowWhatToDo he'll know what to do]]. [[spoiler:It ends up summoning Music/OzzyOsbourne]].
* Happens in ''Film/StarshipTroopers''. A literal ChekhovsGun is handed out when Rasczak shoots and kills a communication officer that has been captured by a flying bug and then exclaims, "I expect any one of you to do the same for me." Later, Rasczak's legs are torn off by a tunneling bug, and he looks at Rico and says, "Rico... you know what to do!" Rico replies, "Yes sir!", and after a moment's hesitation, shoots him.
* [[Film/{{Patton}} "When you put your hand into a bunch of goo that a moment before was your best friend's face, you'll know what to do."]]
[[/folder]]

[[folder:Literature]]
* In PerryMoore's superhero novel ''Literature/{{Hero}}'', the protagonist's mother gives him [[spoiler:a thinly-disguised Kryptonite ring]] with this instruction. Why she doesn't use it herself is less than clear.
* ''Literature/HarryPotterAndTheDeathlyHallows'': [[PowerTrio Each child]] gets a pleasant but [[ItMayHelpYouOnYourQuest seemingly useless gift]] from Dumbledore [[spoiler:by his estate's executor]] with cryptic hints. [[TheChessmaster Each proves critical to their quest]]. Justified because [[spoiler: Dumbledore is dead and since the aforementioned executor takes anything he deems "suspicious"]] he can't outright ''tell'' them.
** It does however raise the question why he didn't just give it to them earlier, transfigured if necessary, since [[spoiler: he knew he was going to die almost a year in advance]]. [[BatmanGambit But hey, it's Dumbledore.]]
** Subverted in ''Literature/HarryPotterAndTheOrderOfThePhoenix''. In the beginning of the book, Sirius gives Harry a package with instructions to use it if Harry needs Sirius' help. Knowing that any attempt to help Harry by Sirius would bring with it an unacceptable risk of being captured and re-imprisoned, Harry resolves never to use it, promptly forgets about it, and doesn't consider that it could be equally useful should ''Sirius'' need ''Harry's'' help. [[spoiler: It turns out to be an enchanted mirror allowing safe, easy two-way communication between the two.]] A lot of tragedy could have been avoided if [[PoorCommunicationKills Sirius had just told Harry what it was instead of being mysterious about it.]]
* In Creator/JohnMoore's ''Bad Prince Charlie'' the [=titular=] prince is given a charm and told 'you'll know what to do when the time comes'. Given what kind of books Moore writes, when the time comes to actually use it as a last resort in the face of an enemy army they mock him by [[LampshadeHanging reciting the same phrase back at him]].
* In David Mitchell's ''Cloud Atlas'', the character Zach'ry in the section "Sloosha's Crossin' an' Ev'rythin' After" is given a series of mystical and seemingly nonsensical clues by a fortune teller, explaining only that he will know what they mean when the time comes. [[spoiler:The clues end up saving his life in the climax, as they are detailed instructions on how to evade a band of savages that is attempting to capture him.]]
* ''Literature/{{Metro2033}}'': Artyom is sent out by the Brahmins (who he's never met) to the Lenin Library (where he's never been) to find a lost book of prophecies (which he's never seen). They totally guarantee that he'll know where it is, though.
* Done several times in the SwordOfTruth. Prophecies given 3000+ years ago can be pretty nonspecific, you know? Additionally, the irrational use of this trope is lampshaded by Richard himself, who, as Seeker, is somehow expected to know how to solve everyones' problems. He actually ends up subverting his own lampshade by being an unusually bright and logical protagonist, and its revealed that one of the primary characteristics of a true Seeker is that they have this trope or the ability to invoke it ''before'' they get the sword.
* In ''TheLordOfTheRings'', Galadrial gives Sam a phial of water from her fountain, saying "May it be a light to you in dark places, when all other lights go out". While this has an obvious literal meaning, it takes on a new significance when [[spoiler:Sam is fighting Shelob and it turns out that the light from a Silmaril is a more potent weapon against the daughter of [[EldritchAbomination Ungoliant]] than the sharpest of swords could ever be]].
* In ''Lovely Assistant'' (by Geoph Essex), when Jenny ''finally'' gets the chance to take a lesson in [[spoiler: [[TheGrimReaper Grim Reaping]]]] from [[TheObiWan Caravel]], Caravel essentially says "YouWillKnowWhatToDo" when it comes to [[spoiler: using the sword and guiding people to the afterlife]]. As it turns out, it's literally true: Jenny instinctively knows what to do when she gets there, and is pretty surprised by that fact herself. (The implication that the experience is better than drugs or sex for [[spoiler: [[TheGrimReaper Grim Reapers]]]] suggests that it's just a "natural" thing for them to do, like fish swimming or birds flying.)
** Near the end of the book, Caravel invokes this trope ''verbatim'' when Jenny is leaving for [[spoiler: her appointment with the [[TheDragon Ice Cream Psycho]]]].
[[/folder]]

[[folder:Live-Action TV]]
* The [[GoldenSnitch amulet]] from the final episode of ''Series/BuffyTheVampireSlayer''.
* ''Series/PrisonBreak'': Linc (when he and Michael go to break LJ out) tells his son "On the third, look out for otis right." and when LJ goes "huh?" Linc answers with the You Will Know What To Do.
* ''Series/{{Merlin}}'' had the Fisher King giving Merlin a waterglobe artifact in season 3 and telling him "When all seems lost, this will show you the way." But Merlin himself had to figure out what to do with it (and did so on accident,dropping and smashing it so the water fell into the bucket and Freya appeared.)
* In the ''Series/{{Supernatural}}'' episode "[[Recap/SupernaturalS06E11AppointmentInSamarra Appointment in Samarra]]", Death instructs Dean to keep researching the souls, and says that he will understand when he needs to.
[[/folder]]

[[folder:Mythology]]
* OlderThanFeudalism: ''Literature/TheBible'' is full of this trope. For example, God directs Abraham to take his son to the wilderness and offer him "in a place I will show you." When he gets there, he finds a sheep waiting to be sacrificed instead.
[[/folder]]

[[folder:Videogames]]
* In ''RatchetAndClank Future'', after a slight Fourth Wall breaking moment, recurring character "The Plumber" gives the protagonists a "3 3/4 Centicubit Hexagonal Washer" claiming it will "[[ItMayHelpYouOnYourQuest come in handy]]". Sure enough [[spoiler:it helps them escape from a collapsing alternate dimension at the end of the game.]]
* Inverted in VideoGame/EscapeFromMonkeyIsland, Guybrush meets...himself from the future! When he does, the future self gives him a few things and says that he'll know what to do with them - only unlike all the other [[ItMayHelpYouOnYourQuest cheap knickknacks that only obtusely solve particular puzzles]], he's actually given ''useful'' things, like a gun and a coil of rope. Guybrush then later uses these items [[spoiler:by giving them to himself in the past, as this is nothing but a puzzle whose solution is to give yourself the items in the exact order you received them in earlier. Deviating from this causes a [[TimeParadox time vortex to open up and take you to the time before you met yourself.]] One of the more amusing ways to screw up this puzzle is by using the gun on your counterpart...]]
-->[[spoiler: Guybrush: "Whoa! I guess it's true that gun owners are nine times more likely to shoot themselves."]]
* In ''VideoGame/EricTheUnready'', you pull the legendary banana "Excalibanana" out of a stone, thus proving that you are the hero chosen to save the princess. You're then instructed to throw it in the pond, so you can magically summon it forth in its time of need. Exactly what use you'll have of a banana, even a legenday one with a badass name, is not readily apparent.
[[/folder]]

[[folder:Visual Novels]]
* In the "Fate" story path of ''VisualNovel/FateStayNight'', Archer takes time [[spoiler:before his HeroicSacrifice]] to tell Shiro that he is a "maker" not a "fighter", and that this will save him. Of course, Archer is [[spoiler:speaking from personal experience, as he's Shiro's older self]].
[[/folder]]

[[folder:Web Originals]]
* Parodied by [[Website/TheBestPageInTheUniverse Maddox]] in [[http://www.thebestpageintheuniverse.net/c.cgi?u=matrix3 his review]] of ''Film/TheMatrixRevolutions'', which he hated. He particularely criticizes the {{Cryptic Conversation}}s between Neo and the Oracle.
-->'''Neo:''' What do I need to do?
-->'''Oracle:''' You KNOW what you need to do.
-->'''Neo:''' Where do I need to go?
-->'''Oracle:''' You KNOW where you need to go.
-->'''Neo:''' Who do I need to see?
-->'''Oracle:''' You KNOW who you need to see.
-->'''Neo:''' What do you want?
-->'''Oracle:''' The same thing you want.
-->'''Neo:''' How will I know I'm making the right choice?
-->'''Oracle:''' You will KNOW...!
[[/folder]]

[[folder:Western Animation]]
* Parodied in ''WesternAnimation/FamilyGuy'', where Quagmire is told this and given... [[ItMayHelpYouOnYourQuest a banana]]. Further subverted when Quagmire is running for his life, recalls the cryptic advice, and throws the banana at his attacker. It bounces off harmlessly, falls to the ground, and both parties look expectantly at it. Nothing happens, and after a moment, the chase resumes.
** Of course nothing happens, because Quagmire was too GenreBlind to do a banana peel gag. [[spoiler: Cleveland]] was probably looking at it less expectantly and more "Did he just throw a banana at me?"
* In one episode of ''WesternAnimation/XiaolinShowdown'', Omi is given the puzzle box that is the key to resealing Wu Ya, but isn't told how it works, save that, "it will open when the person who needs to open it opens it."
-->'''Dojo:''' I know. This kinda stuff drives me crazy too.
* Episode 52 of ''ScoobyDooMysteryIncorporated'': The Annunuki, in the host body of Nova (Brad and Judy's Cocker Spaniel), tells Scooby this during the final battle with the Nibiru Entity. Scooby gets very frustrated and tells her to ''just tell him'' what to do since they are in such danger.
[[/folder]]

[[folder:Real Life]]
* Common with sex and romance. No one will tell you how they work, but it's assumed that you'll know what to do.
[[/folder]]
----