Someone's looking at a spider and thinking, "I really don't want this to bite me...oh wait! Strange markings. Maybe it's radioactive! Oooh, maybe I'll get superpowers!"
So he lets it bite him. Then he waits a minute, and nothing happens, so he tries to shoot a web from his hand. "Spidey powers! C'mon, spidey powers!" Then his hand falls off. "Yeah...not the reaction I was hoping for."
At least, I THINK it's funny. I hope, anyway. It's something my brother and I came up with a while back. ^.^