I use this topic for filler or for off book days in all of my Elementary classes.
Basically I write the names of various animals on the board, and ask students, "Who would win in a fight? Why?"
The more varied the animals, the funnier the answers. The only parameters I put up are they must speak only English and explain their answer. It's great to see their little brains nearly implode trying to think of how to explain why monkey would beat a mouse.
This is incredibly easy to make more difficult or to simplify. You can really stretch it for class time as well.
