The girl has been watching him and says, "You must be a dentist."
The guy, surprised, says "Yes....how did you figure that out?"
"Easy," she replied, "you keep washing your hands."
One thing led to another and they make love.
After they have done, the girl says, "You must be a very good dentist."
The guy, now with a boosted ego says, "Sure, I'm a good dentist, how did you figure that out?"
Spoiler: