It's pre-class converations like these that make me seriously consider just showing up 5 minutes late for everything.

Undergrad, out of the blue: Are you dating Claire?
Me: Who's Claire?
Undergrad: She was in this class last week. She sat in the corner (indicates a corner of the table) over there.
Me: I don't think I know Claire.
Undergrad: (silence)
Me (helpfully): It's probably good that I'm not dating her, since I'm already dating someone who's in law school at Berkeley.
Undergrad: Oh.
Me: It would be a shame if I were to cheat on her behind my own back.
Undergrad: (stony silence)