A young boy walks by his parents room and catches them having sex.
The next morning he says to his mother"Why were you jumping up and down on dad last night"
Thinking quickly she says"You know the way your dad has a big belly,well i was jumping up and down on it to flatten it out"

The boy looks at her and says"Well thats just a waste of time"

Puzzeled the mother asks "why"

The boy says
"Well every night you flatten it,but when you go to the shops Mary from next door comes in and blows it up again"

kids don't miss much