Classical conditioning is learning through association. For example, Pavlov conditioned dogs to salivate when they heard the sound of a bell by associating food with the sound of the bell. On the other hand, operant conditioning is learning through consequence and reward. This includes positive reinforcement, rewarding every time the desired behaviour occurs, and negative reinforcement, removing or stopping a negative stimulus. For example, BF Skinner used both types of reinforcement in his classic experiment using rats in a box; positive reinforcement occurred when they pulled the lever and food pellets were released, and negative reinforcement occurred when they pulled the lever and the electric current running through the floor was stopped.