The son sees his mom 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 mom, puzzled.

"Well when you go shopping the lady next door comes over and gets on her knees and blows it right back up."