What goes in a gas tank?

Kyler and I were in the car this morning. He saw someone who had left thier gas tank door open, and said that was bad. Then he asked why gas tanks need a cover. I said that you didn't want the gas to splash out, or anything else to go in.
K: Like what?
M: You don't want rain, or water, or anything else to go in.
K (totally serious): Like a sandwich could accidentally fall in.
M: Yeah, that would be bad...