"Noises at Night" joke

This 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!"