A man and his girlfriend are having a sexual encounter.
He asks her to "go downtown" so, with a sigh, she gets on her knees in front of him and starts peering at his genitals, looking and tipping her head this way and that, studying the whole business.
After about five minutes of this, he asked her in a sort of peeved voice "Well, just what are you doing?"
She said "I'm doing what I always do when I'm downtown with no money!!