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 super smart.
You knew about the Time-Turner before the movie came out and then rolled on the floor like a lunatic.
You told your class about the Horcruxes and then laughed like you just won the lottery.
You spoiled the whole series for your cousin and then cackled like a madman.