A little boy wakes up 3 nights in a row when he hears a thumping
sound coming from his parents room. Finally one morning he goes
to his  mom and says, "Mommy, every night I hear you and daddy
making noises and  when I look in your bedroom you're bouncing
up and down on him."

His mom is taken by surprise and says, "Oh... well...ah....well
I'm bouncing on his stomach because he's fat and that makes
him thin  again."

And the boy says, "Well, that won't work!" His mom says,

"Why?!?"

And the boy replies, "Because the lady next door comes by
after you leave each day and blows him back up!"