What are the rules Bilbo and Gollum agree to concerning the riddle contest?

The rules are simple, if Bilbo can't answer the riddle, Gollum get to eat him. But if it goes the other way, Gollum must show Bilbo the way out.

If precious asks, and it doesn't answer, we eats it, my preciousss. If it asks us, and we doesn't answer, then we does what it wants, eh? We shows it the way out, yes!"