J.K. Rofling
A fake fit of laughter that happens when you know something from the Harry Potter books and then act like you're the king of the world.
You knew about the Horcruxes before the book came out and then laughed like you just ate a thousand candies.
You told your class about the Time-Turner and then rolled on the floor like a madman.
You leaked the final battle and then cackled like a witch on a broomstick.