Although behavioral psychologists use many different kinds of equipment in operant conditioning experiments, one device that is frequently employed is the Skinner Box. The box, named for B. F. Skinner, the American psychologist who developed it, was used in Skinner's original operant conditioning experiment in 1932. The Skinner Box is a small, empty box (5) except for a bar with a cup underneath it. In Skinner's experiment, a rat that had been deprived of food for twenty-four hours was placed in the box. As the hungry animal began to explore its new environment, it accidentally hit the bar, and a food pellet dropped into the cup. The rat ate the pellet and continued exploring for more food. After hitting the bar three or four times with similar result, the animal learned that it could get food by pressing the bar. The food (10) stimulus reinforced the bar pressing response. The rat had been conditioned.

The word "it" in line 5 refers to