Define 'flirting'.

I say seriously, "Gatorade makes my mouth dry." --insert sad/frowny face here--

He jokes, "DRY? What are you putting in your mouth that makes it dry? Do you need something to hydrate? Some service? Cuz I can't do that for you; I already have a girlfriend."

I give him a sarcastic look. "I wasn't thinking in that direction at all."

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Ok, so, you guys, I need some help. As if that little excerpt up there wasn't enough to clue you in, I am a little inexperienced with the whole boy-girl romance thing. (yes, I do write romance. It's so much easier to fake than actually experience it in real life)

I'm not exactly sure what flirting is.

Is it when he teases you? Tries to make you laugh constantly? Twist everything you say into something perverted so he can keep the conversation going?
Repeatedly say over the course of two hours, "It's so weird that I've never seen you before. How many years have you been going here?" ..... "Four? Really? You've never been in anyyy of my classes. I can't believe I never met you before this."
Or when you comment on how short he is so he bumps against you and steps up onto the sidewalk next to you and is suddenly 6 inches taller and repeats what you say, "You're short." and then proceeds to muss your hair a little with a goofy smile all over his face?

Cuz he's done all of that to me. And I've never really noticed when, or if, a guy is flirting with me. But this time, I got the feeling that he was.

In conclusion, I'd like your guys' takes on what is flirting. Oh, and maybe some advice as what I should do next. (He has a girlfriend and I know he's not a cheater.)