A couple were in thier bed ready to make love when their four year old son walked in an asked, "Daddy can I ride on your back?"

The Dad said, "NO!".

The wife told her husband, "It's OK honey, he doesn't know whats going on." so he agreed.

As they got in to the throws of passion and the wife started moaning, breathing heavy and thrashing about.

The little boy shouts, "Daddy you better hold on! Yesterday when she was doing that me and the Milkman got thrown off the bed."