Biff is 6'3" tall, and the first few times he enters or exits a commuter airplane, he bangs his head on the ceiling. Biff now crouches when entering or exiting a commuter airplane, attesting to the usefulness of classical conditioning.

Biff has learned to associate the experience of hitting his head with entering or exiting the airplane, and has therefore modified his behavior to avoid that negative experience by crouching. This is an example of classical conditioning, where a stimulus (entering/exiting the airplane) becomes associated with a response (hitting his head) and leads to a change in behavior (crouching).