Stomach bouncing

This little boy wakes up 3 nights in a row when he hears thumping coming from his parent's room. He goes to his mom and says, "Mommy, every night I hear you and daddy making noise and when I look in you're bouncing up and down on him."
His mom is taken by surprise and says, "Oh... well I'm bouncing on his stomach because he's fat and that makes him thin again."
The boy says, "That won't work."
His mom says, "Why?"
The boy replies, "Because the lady next door comes by after you leave each day and blows him back up!"