Oral sex

A man is at the dentist's for a check-up. As the dentist leans over, he asks, "Well... So you had oral sex this morning?"

"How did you know?" asks the man, embarrassed but also amazed at his dentist's perception. "Was it the smell on my breath?"

"No" says the dentist.

"Well, did you see a pubic hair caught in my teeth?" asks the man.

"No" says the dentist.

"Well, what then? How did you know?" asks the man, losing patience.

The dentist says "There's a little bit of shit on the end of your nose."