Sorry if this is too risque:

A six year old walks into the kitchen where his Mom is preparing a meal and says, "Mom, the last few nights I woke up to this thumping noise coming from your bedroom and when I went to see what it was you were sitting on top of Dad and bouncing up and down. Why were you doing that?"

The startled mother recovers quickly and replies, "Your Dad is overweight, and I am trying to get him back down to normal size. I bounce on him to get all the air out of him."

The little kid just shakes his head and says, "Mom, you're wasting you time."

The Mother asks, "Why is that, dear?"

The kid says, "Because, in the morning after you leave for work that nice-looking lady next door comes over and blows Daddy right back up again!"