Operant conditioning (also called instrumental conditioning) involves an organism's learning to make a response in order to obtain a reward. The response is an action not typically associated with obtaining a particular reward. B.F. Skinner pioneered the study of operant conditioning, altough the phenomenon first was discovered by Edward L. Thorndike, who proposed the law of effect, which states that a behavior is more likely to recur if reinforced. Skinner ran many operant-conditioning experiments. He often used a specially designed testing apparatus known as a Skinner Box. This box typically was empty except for a lever and a hole through which food pellets could be delivered. Skinner trained the rats to press the lever (not a typical behavior for rats) in order to get food. To get the rats to learn to press a lever, the experimenter would use a procedure called shaping, in which a rat first receives a food reward for being near the lever, then for touching the lever, and finally for pressing the lever. In the end, the rat is only rewarded for pressing the lever. This process is also referred to as differential reinforcement of successive approximations. In a typical operant conditioning experiment, pressing the bar is the type of response (also called an operant), and food is the reinforcer. Food is a form of natural reinforcement that doesn't need to be learned to be reinforced. These types of natural reinforcers, such as food, water, and sex, provide primary reinforcement. Secondary reinforcement is provided by learned reinforcers.