Ivan Pavlov (1849-1936) was the first to show the way in which it works. He did this in an experiment using dogs. Pavlov noticed that the dogs naturally salivated when they saw food. This behaviour did not need to be taught. In this example, the food is an "unconditioned" stimulus. He paired this unconditioned stimulus (showing food to the dogs) with another, "neutral" stimulus: the ringing of a bell. Pavlov discovered that, if the two stimuli are presented together again and again, the organism "learns" that they belong together. As a result, it is enough to show the neutral stimulus to get a conditioned response or reflex.