Timmy comes home early...

...and walks inside to see his mom on top of his dad and both are naked. Timmy just stares, confused what is happening or going on.

Mom: "Timmy, what's wrong? Go to your room, I'll be up in a bit"

So Timmy goes upstairs to his room. Later his mom comes up and sits down next to him.

M: "Timmy, what did you see?"

T: "I saw you and dad naked and you were on top of him jumping up and down."

So, Timmy's Mom brainstorms what to tell him, since she doesn't want to admit and explain sex. Then, an idea came to mind.

M: "Is that all you saw, you did not know what we were doing?"

T: "That is all I saw and I have no idea."

M: "Well, Timmy, when you get older like your father and I you need to exercise. When your fathers belly gets too big and full of air, we get naked and I go on top of your father. Then I push my hands on his gut, jumping up and down to take all the air out to make him skinny again."

T: "Mom, you know that is pointless, right?"

M: "What do you mean?"

T: "When I am at the bus stop in the morning, waiting for the bus to come and after you leave for work, the next door neighbor Susan comes over and blows him back up!"