stop laughing, start clapping, an almost rhyme

I was teaching a class yesterday for about 40 college seniors. The class is set up so that I couldn't walk around amongst them at all--I had to stand in front of them in good old lecture format. So I had lots of time to look at people's faces while I was talking and, as usual, there were a couple of guys who were smiling and sort of laughing. I don't get that sort of reaction from girls, who would surely be the ones smirking if my fly were hanging open, my hair were even crazier than usual or my sweat stains were reaching gargantuan proportions.

I was left wondering, "What the hell is so funny?"

If you happened to see me yesterday and you have the answer to this question, please post it in the comments because I'm super curious.