This is kind of naughty on my part, but here we go:
During halloween, there were a few decorations about the place. Generic cartoony skeletons and pumpkins, mostly, with the odd spider. Pretty much entirely in large shops and supermarkets, since otherwise no-one does anything for it in this country. But one kid I teach got fixated on seeing a skeleton somewhere, and was teasing other kids by saying that a skeleton was hiding in their bedrooms and was going to "get" them, etc.
So I told the kid: "Did you know that there's a skeleton hiding in this very room?"
His face went pale. "Really?"
"Oh, yes. Would you like to know where it's hiding?"
Looking very nervous now. In a small voice, "... yes."
"It's hiding... inside you!"
He got half-way through a shriek before stopping, mouth still open, when he furrowed his brow and said, "Hang on a second..."
To see someone's expression change from screaming terror to "huh?" is probably one of the best things ever. He stopped bugging other kids after that, at least.