Flatten It or Blow It Up

A little boy walks into his parents'
room to see his mom on top of his dad
bouncing up and down. The mom sees her
son and quickly dismounts, worried
about what her son has seen. She
dresses quickly and goes to find him.
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."