Slappy: Hey! Isaac Newton. Interesting bit of trivia for you.
Stinkbomb: What's that?
Slappy: Dogs can't climb trees.
Nature is funny sometimes. In media, people may actually be able to briefly perform a task they naturally can't for the sake of comedy until they are reminded that they don't have the ability to perform the action in question. The reason they start performing an impossible action in the first place is that they are so determined to achieve it that they forget they can't do it.
Of course, once they are reminded that they can't, they may immediately give an Oh, Crap! expression before Reality Ensues. Any moment where somebody does something that shouldn't even be possible applies if it gets pointed out and they immediately stop doing it or it stops working.
May be subverted if the reminder is there and the character involved still goes on doing their improbable thing.
Often overlaps with Gravity Is a Harsh Mistress when the thing they forgot was gravity or the end of the ground beneath them.
Compare Centipede's Dilemma, which involves a question on their ability rather than a genuine fact on their inability, Forgot I Couldn't Swim, when a character remembers their inability to swim upon jumping into water, This Cannot Be!, and Puff of Logic, which points out their inability to live or even exist. Compare Achievements in Ignorance, which tends to set up this trope, and contrast Forgot I Could Fly.
- Soaky was a 1950s/60s kids' bubble bath in containers resembling cartoon characters whom themselves appear in the TV commercials for it. One spot featured Superman and Tennessee Tuxedo. In it, Tennessee is in midair flapping his hands as Superman flies up next to him:
Superman: Tennessee, penguins can't fly!
Tennessee: I happen to be skydiving.
Superman: Without a parachute?
Tennessee: (plummeting suddenly) I knew I forgot something! (Superman catches him)
- In Looney Tunes: Back in Action, there's a moment where the protagonists are falling out of the sky in a spy car that is low on fuel. Mere feet before they hit the ground, everything comes to a stop, which Bugs Bunny handwaves as the car being out of gas. When Kate, who is shown to have no sense of understanding of Toon logic, points out that reality doesn't work like that, the car promptly crashes into the ground.
- In The Pirates! In an Adventure with Scientists!, The Pirate Captain boards a ship only to find it's a ghost ship. Once he realizes the ship isn't solid, he falls right through it.
- Bunnicula: Harold the dog is fond of eating chocolate, cheese, and fudge. He is reminded in some books that these foods are poisonous to dogs, but he remarks that he is a fictional dog and thus can eat anything he wants.
- The Hitchhiker's Guide to the Galaxy: One book explains how to fly by throwing yourself on the ground and missing. One rule is never to listen to what people on the ground are saying, because they will invariably tell you this is impossible, which will cause you to fall. Arthur Dent finds out this is true, since he starts to fall out of the sky when he thinks about how impossible it is for him to fly.
- On Cheers, Coach got excited on started doing handstand pushups like in the old days. Sam reminded Coach that he never did handstand pushups, that was his teammate. Coach immediately fell down.
- An episode of Gilligan's Island has the title character manage to fly with fake wings. When Skipper points out this is impossible, Gilligan instantly crashes.
- Disney Adventures: In one arc of comic The Hair Pair, the starring rodents are cornered by their nemesis, Maxwell Blowfish. The villainous fish has them cornered, until they question how he's breathing out of water. Immediately, Maxwell starts suffocating, allowing the two to escape.
- Animaniacs is a big fan of this.
- In one Slappy episode, Slappy and Skippy are dealing with a dog named Stinkbomb. After climbing their tree, Stinkbomb follows them up. Leave Slappy to bring up one important fact that Stinkbomb forgot about. Dogs can't climb trees.
- One instance of Buttons protecting Mindy has him following her up a tree. After she tells him that dogs can't actually climb, he immediately falls.
- The Critic: When Franklin Sherman discovers that the pilot of his plane was a penguin (who has been drinking on the job), he remembers that penguins can't fly. The plane immediately goes into a tailspin.
- Garfield and Friends:
Garfield: Boy, with some people, you have to tell them everything.
- One episode has Garfield getting chased by a dog. At one point, the chase has them on the ceiling. During that moment, Garfield stops to ask if the dog has sharp claws. When the dog shakes his head, Garfield tells him that he can't be on the ceiling because of that causing the dog to fall.
- Lampshaded during a U.S. Acres segment. Orson points out that, in the real world, if you stepped off a cliff, you'd fall into the chasm the exact moment your feet stepped into thin air — whereas, in a cartoon, if you do the same thing, "you don't fall until you realize you're falling".
- The Grim Adventures of Billy & Mandy: When Billy and his dad enter a metal competition, Billy's dad decides to make a flashy entrance by flying onto the stage. However, the moment Billy says "I didn't know you could fly," his dad immediately realizes what Billy is saying and falls.
- Looney Tunes: In the short "Birdy and the Beast", Tweety jumps off a tree to fly away from a hungry cat, and the cat jumps off the tree to fly after him. Of course, he plummets once he's aware of what he's doing.
Tweety: Gee whiz, puddy tat, I didn't know you could fwy.
Cat: [laughs] You didn't know I could fly. Heh-heh... FLY?!?!?!
- The Moxy Show: In the one episode produced for The Moxy and Flea Show, Moxy is driving a car with Flea when the latter starts to talk about how it bothers him that dogs can't drive. It takes a while for Moxy to be thrown off by the realization that he's a dog driving a car.
- SpongeBob SquarePants:
- This classic scene:
SpongeBob: At least it's warm around the fire.
Patrick: Hey, if we're underwater, how can there be a —
[fire goes out]
- In "House Sittin' for Sandy", SpongeBob spends a good part of the episode in Sandy's treedome without his helmet. It's only when Patrick shows up wearing his and SpongeBob inquires about it does Patrick remind him they can't breathe air and he starts choking.
- This classic scene: