Dialogue where characters awkwardly tell each other things they both already know, purely to inform the reader.
As You Know, Bob is a dialogue antipattern where one character explains information to another character who logically already has that information. The name comes from the classic construction: "As you know, Bob, our planet has two suns." It's a shortcut writers use to sneak exposition into conversation, but it breaks believability because real people don't explain common knowledge to each other.
Readers are perceptive. The moment a character starts explaining something the other character clearly already knows, the illusion cracks - you can practically see the author pulling the puppet strings. Learning to spot and fix AYKB in your own writing forces you to find cleverer, more natural ways to deliver information.
Characters regularly explain the political situation of the Republic to each other - senators explaining Senate procedure to other senators - because the audience needs the context.
Robert Langdon, a world-renowned symbologist, frequently has basic symbolism and history explained to him by other experts, serving as a vehicle for reader education.
"Don't you remember that our planet has two suns?" is still AYKB with a question mark. The character still wouldn't need to say it. Rework the scene so a newcomer, child, or outsider can naturally ask.
Having one character who "needs everything explained" can work early on, but if they're still getting tutorials in Chapter 20, it strains belief. Let your newcomer learn.
The fix isn't moving the exposition from dialogue to a narrative paragraph dump. It's weaving information into action, conflict, and context so readers absorb it naturally.
Find a scene in your current project where one character explains something the other character should already know. Rewrite the scene three ways: once with a newcomer character who genuinely needs the explanation, once using only action and environmental details to convey the information, and once by cutting the exposition entirely. Which version reads best?