Little Johnny 7

Little Johnny walked into the kitchen, saw his mother making a cake and announced, "I'm gonna go play in my room for a couple of hours.  I sure would like a piece of cake after though!

Later, when his mother brought him a piece of cake, Little Johnny exclaimed, "Wow!, it worked!"

Puzzled, his mother asked, "What do you mean?"

Little Johnny replied, "Daddy said that in order to get a piece around here, you have to spend a couple of hours playing
first!"