Timmy comes home early and walks in on his parents, her mom explain it to him, but there's a contradiction.

One day, Timmy comes home early from school and walks inside to see his mom on top of his dad – both naked. Timmy just stares, confused as to what they’re doing.

The mom says, “Timmy? 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.

She asks, “Timmy, what did you see?”

Timmy replies, “I saw you and dad naked, and you were on top of him, bouncing up and down.”

So, Timmy’s mom thinks about what to tell him, since she has to go grocery shopping and doesn’t have the time to explain sex right now. Then, an idea comes to her mind.

“Is that all you saw, you have no idea what we were doing?”

“That’s all I saw and I have no idea.”

“Well Timmy, when you get older like your father and I, you need to exercise. When your father’s 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, bouncing up and down to take all the air out to make him skinny again.”

Timmy starts laughing, so his puzzled mother asks him, “What’s so funny?”

Still giggling, he says, “That’s not going to work! Every time you’re gone, the babysitter just blows him back up again.”