Here's a puzzle I picked up from a BBC puzzle show. Romeo is getting ready for a date with Juliet. He figures it'll cost him 30 pounds and he has 20. He goes to the pawn shop and borrows 15 pounds; he gives the 20 pound note as security. Back at the dorm, he sells his pawn ticket to Roomie for 15 pounds.
The question is: who bears the greatest cost in this transaction, and how much is it?
This one is said to have been devised by Lewis Carrol, he of Alice in Wonderland fame. For an answer to the puzzle, and for a meta-puzzle, go here.