Just think of the possibilities.
A personal secret: when I was younger, I trained dogs for obedience competitions. It was, if you'll pardon the obvious pun, a rewarding experience. I loved the process of introducing a new concept to the animal, then watching it struggle, grasp, and finally understand and respond appropriately.
Humans are a wilier subject, though we are by no means untrainable. In fact, it's remarkable what you can accomplish using just basic conditioning. Here are two videos that showcase exactly that. The first introduces a new stimulus a la Pavlov, the second relies on an pre-existing stimulus to solve the age old problem of being stuck behind slow walkers. Enjoy!