A little girl and her mother were out walking when they passed two cats having sex.
"Mummy, what are those cats doing?"
"The fat kitty is pumping up the thin kitty, darling."
"Why's it doing that, mummy?" asked the child.
"Because the thin kitty is sick and pumping her up will make her better."

The next day, the little girl was out walking with her mother again when they passed two dogs having sex.
"Mummy, what are those dogs doing?"
"The big doggie is pumping up the little doggie, darling."
"Why's he doing that, mummy?" asked the child.
"Because the little doggie is sick and pumping him up will make him better."

The next day, the little girl was out walking with her mother again when they passed a bull mounting a cow.
"Mummy, what's that bull doing to that cow?"
"The Bull is pumping up the cow, darling."
"Why's he doing that, mummy?" asked the child.
"Because the cow is sick and pumping her up will make her better."

That weekend, the little girl came rushing into the kitchen and shouted:
"Mummy, mummy, I think the au-pair is sick!"
"Why?" replied her mother, in alarm, "Whatever is she doing?"
"She's lying on her stomach on your bed and Daddy is on top of her, pumping her up!"