Tommy's house is packed with relatives for Christmas dinner.  Grandpa calls 6
year old Tommy and starts asking about school, girlfriends and other stuff he
can think of.  After a while, grandpa notices that Tommy is losing interest in
the conversation so he pulls out two bills from his wallet to see if he can keep him interested.  A ten and a twenty-dollar bill.  He shows both bills to Tommy and tells him that he can keep any one he chooses.  Tommy reaches over and grabs the ten-dollar bill.

Grandpa pretty surprised and upset about the unwise decision his grandchild made, pulls out another ten dollar bill to see if it was a mistake.  Again, he tells Tommy to take one of the bills and keep it. Tommy grabs the other ten. Grandpa again is surprised and upset.  He takes Tommy over to one of the uncles and shows him how dumb Tommy is in choosing the ten over the twenty.  Grandpa goes on and on showing every uncle and cousin and each time Tommy chooses the ten over the twenty.  Grandpa finally shows the stunt to daddy.  Daddy's quite surprised but doesn't pay too much attention at the moment.

A few hours later, daddy who is very concerned about Tommy's poor decision, walks up to him and asks him if he knows the difference between a ten-dollar bill and a twenty. 

"Of course," answers Tommy. 

"So why did you always choose the ten over the twenty?" asks dad. 

Tommy, with a wide smile answers, "Well dad, if I would have chosen the first twenty dollar bill, do you think grandpa would have played the game fifteen more times?"