A Flat Stomach 
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 and help flatten it..' 
'You are wasting your time,' said the boy. 
'Why is that?' the mom asked puzzled. 
'Well when you go shopping the lady next door comes over and gets on her knees and blows it right back up again!!