Ivan Pavlov was a Russian psychologist who developed the theory of classical conditioning based on his experiments with dogs in the late 19th/early 20th century. He found that dogs could be conditioned to salivate in response to a stimulus other than food, like the ringing of a bell. Specifically, he noticed the dogs began salivating when they heard the bell ring, as that was when they were usually fed. He then demonstrated that this new response was learned by conditioning the dogs to salivate to a different bell. Pavlov's work showed how stimuli in the environment can become associated with innate responses through repeated experiences.