You wander around the cage, and, passing the bar, you realize that the tin shield is missing. You accidentally press the bar, and immediately a pellet of food is released. Wonderful! You press the bar again. Oh joy!—another pellet comes out. You press again and again, eating happily, but then the light goes off, and at the same time, the bar stops releasing food pellets. You soon learn that when the light is off, no matter how much you press the bar, you don’t get any food.
