When a guy is staring at a girl, what doe he think about?

To be honest, depending on the chick, id be thinking about how HOT she is (Much like admiring a nice painting). That is, if I wasn't just day dreaming and not paying attention to where my eyes were left. Much like admiring a nice painting.

It can be that he's not thinking about you at all. Sorry, but sometimes when I'm staring at a girl, I'm not thinking about her at all - I'm just lost in my own world. Sometimes guys are accused of not paying attention, and yet they've been watching and 'listening' the whole time - it's because he's just lost in his own mind and can't hear you at all.
When he IS thinking about you (most of the time, despite what I've just said), it is a little more, erm, erotic, just as CaitsEM said. We're thinking about her body, her eyes, her clothes and how it makes her look overall. As much as 'undressing her with my eyes' has been popularized, that's rarely the case. I usually am thinking about how pretty she is, not what she looks like naked unless she actually is naked or she is leaving very little to the imagination.

The thoughts are as varied as there are unique people on this planet. Some thoughts may be quite erotic whereas some may be as innocent as him dreaming about gazing into your eyes while walking you home.

I can't possibly answer this question in full to someone who may be below the age of consent.

Watch several pornos, and you'll begin to get the idea of what we're working with here, by and large. The younger you are, the more innocent it is, I'd say.

I obviously don't have first hand experience with this, but. . . Well, what are YOU thinking about when YOU stare at HIM?! Take that, then make it more erotic.