This little boy wakes up 3 nights in a row when he hears thumping coming from his parents bedroom. Finally one morning he goes to his mom and says,

"Mommy, every night I wake up and hear you and Daddy making noises and when I look into your room your bouncing up and down on him."

His mother replies with, "Oh,..well...ah...well I'm bouncing on his stomach because he's fat and that squishes his belly down and makes him thin again.." The boy replies, "Duh, mom, that won't work because the lady next door just comes over every day and blows him back up.