Here's a good one to tell your kids:
3 guys walk into a hotel and each pay $10 for a total of $30 for a room. the clerk at the front desk realizes he overcharged them $5 so he summons the bell hop and tells him to take the $5 up to the room.
He goes to the room and explains to one of the three friends that they were overchrged and gives him the $5. Realizing he can't easily split $5 between the three of them he gives the bellhop a $2 tip. So each friend gets a dollar back and spent a total of $9 for the room.
3 x 9 = 27
plus the 2 the bellhop has = $29 , Where did the dollar go?