Ivan Pavlov (1849-1936) was a Russian physiologist who won the 1904 Nobel Prize in Physiology or Medicine for his famous experiment in which he trained dogs to salivate at the sound of a metronome. The experiment helped lead to the development of the behaviorist theory of psychology and is perhaps the "textbook" example of classical conditioning today.
Pavlov's experiment for which he is most well known started out as something entirely different. As a part of a study on digestion that Pavlov was conducting on dogs, Pavlov rang a metronome every time he fed them. He noticed that the dogs would begin to salivate at the sound of the metronome, even if no food was present. He also found that if the metronome was rang without food enough times, the reflex would become extinguished. In other words, the dogs would stop salivating at the sound of the metronome.