The puzzle box is the laboratory device that E. L. Thorndike invented in order to study instrumental or operant conditioning in cats. Hungry cats were individually placed into a box that could be opened by the animal via a device such as a latch. Once outside of the box, the cats gained access to food (a positive reinforcer). Thorndike found that the cats took less and less time to get out of the box the more trials of training had been given. He referred to this reinforcement of latch opening as documenting the Law of Effect.