A little boy wakes up three nights in a row when he hears a thumping sound coming from his parents bedroom. Finally, one morning he goes to his mother and says, "Mummy, every night I hear you and daddy making noise and when I look in you're bouncing up and down on him."
His mum is taken by surprise and says. "Oh ... well I'm bouncing on his belly because he's fat and that makes him thin again."
The boy says, "That won't work."
His mum says, "Why?"
The boy replies. "Well because Sylvia next door comes in after you leave each day and blows him back up again!"