Ivan Pavlov pioneered the study of classical conditioning, the use of external stimuli to elicit learned responses. According to PBS, he is famous for experiments that involved training dogs to salivate in response to the sound of a metronome.
Ivan Pavlov's work laid the foundation of classical conditioning. The framework underlying this technique is that subjects can be trained to manifest a "conditioned" response by pairing a biological component, or "unconditioned" stimulus, with a neutral element, or "conditioned" stimulus.
Pavlov's own experiment with dogs is a perfect example of classical conditioning. He found that dogs salivated when exposed to food. He began to sound a metronome when feeding the dogs. Eventually, the dogs would salivate at the sound of the metronome even when there was no food present.