A woman goes to the doctor for an examination.

After
thoroughly examining her, the doctor says, "You are in
excellent health except for those bruises on your knees.
They seem to be developing into calluses. Can you tell me
where they are coming from?"

The young woman starts to blush and says, "I guess they have
to do with my sexual activity. Whenever we make love we do
it doggie style."

"Well that should be easy to take care of," says the doctor,
"surely you must know other ways to have sexual intercourse."

"Oh! Yes, I do," says the woman, "but my dog doesn't."