One day a little boy walks in on his parents having sex. He looks at his father and says, "Daddy what is that?" The father says, "Oh son that is....daddy's lawn mower!"
Then the boy says, "Daddy what's that between mommy's legs?" The father hesitates for a moment then says, "Well son, that's mommy's lawn!"
The little boy looks at his father again and asks, "What were you doing?" Then the father says, "Well I was mowing mommy's lawn!" The boy looks confused and says, "Well her lawn must grow fast cause the mailman just mowed it yesterday!"