Kid Catches Parents

A little boy walks into his parents' room to see his mother on top of his father bouncing up and down. The mother sees her son and quickly dismounts, worried about what her son has seen. She quickly dresses and goes to find him and, when the son sees his mother and asks, "What were you and Dad doing?" 

The mother replies "Well you know your dad has a big tummy and sometimes I have to get on top of it to help flatten it." 

"You're wasting your time," said the boy. 

"Why is that?" asked his Mother, puzzled. 

"Well, each time you go out shopping, the lady next door comes over and gets on her knees and blows it right back up again!"