"Chuck Jones on Wile E. Coyote & Road Runner, from Chuck Amuck: Here were some of the rules we obeyed in the Coyote-Road Runner series:
Rule 1. The Road Runner cannot harm the Coyote except by going "Beep-Beep!"
Rule 2. No outside force can harm the Coyote - only his own ineptitude or the failure of the Acme products.
Rule 3. The Coyote could stop anytime - if he were not a fanatic. (Repeat: "A fanatic is one who redoubles his effort when he has forgotten his aim." - George Santayana) "
See the rest of the list here.