While working as a research fellow at Harvard, B. F. Skinner carried out a series of experiments on rats, using an invention that later became known as a “Skinner box.” A rat was placed in one of these boxes, ① which had a special bar fitted on the inside. Every time the rat pressed this bar, it was presented with food. The rate of bar‑pressing was ② automatically recorded. Initially, the rat might press the bar accidentally, or simply out of curiosity, and as a consequence ③ receive some food. Over time, the rat learned that food appeared whenever the bar was pressed, and began to press ④ it purposefully in order to be fed. Comparing results from rats ⑤ gives the “positive reinforcement” of food for their bar‑pressing behavior with those that were not, or were presented with food at different rates, it became clear that when food appeared as a consequence of the rat’s actions, this influenced its future behavior.