spam...who is responding to it?

I've long been fascinated by the sort of strange, and occasionally poetic (The Tornado Boys of 33) stuff that ends up in my junk mail folder. And you get used to over time. But that doesn't make it any less disturbing to get an email from "Grandma" with the subject line "GetNasty". No thanks, Grandma!

I'm very curious to know what the returns are on spam. I have trouble believing that it is worth anyone's effort to send it. But maybe I'm overestimating the average internet user's intelligence? I am an optimist after all.