On the way home on the train, a guy hit on me. It happens so rarely that it surprises me and I'm never sure how best to respond.

I don't know why you should surprised. Were you not off the market, I would hit on you. But then again, we've had that conversation before. :p

When I was an angst-ridden teen, Mom assured me that boys weren't asking me out because I intimidated them (me?!?).

You.... Intimidating.... Now that's amusing. Your even cute when your mad.