Little Johnny walked into the kitchen, saw his mother making a cake and 
announced, "I'll be playing in my room for the next two hours.  I sure would 
like a piece of cake when you're finished."

Later, when his mother brought him a piece of cooled cake, Little Johnny 
exclaimed, "Gee, 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!"