Three men check into a hotel late at night. There is only one room left, it costs $30, and the men decide each to chip in $10 and to share the room.
When they get to the room, they find that it is a disgusting mess. The room was never cleaned after the last people checked out. The men call the manager to complain.
The manager agrees to knock $5 off the price. He sends a bellboy to the room with clean linens and the $5 refund in the form of five $1 bills.
The three men each take $1, and let the bellboy keep $2 as a tip.
So, each man originally paid $10 and got $1 back, which means that each paid $9, which sums to $27, and the bellboy has $2, which all sums to $29.
So, where is the other dollar?
(From Everything and More, by David Foster Wallace)