I was very amused to overhear this conversation between a small child and her mother on the tube:
"Mummy, when are we going underground?"
"We ARE underground."
"Don't be silly, no we're not."
"Yes we are. Look up - you can't see the sky, can you?"
"No, that's because the roof of the train is in the way."
"Anyway, if we were underground we'd be able to see moles through the window."