Conditioning occurs when an animal's behavior changes as a patterned response to a certain stimulus. The founder of classical conditioning was Ivan Petrovich Pavlov, who devoted more than 30 years to its study. Pavlov began his research career investigating digestion, winning a Nobel prize in 1904 for his work. Pavlov began his signature work on dogs in 1902. He restrained a hungry dog on a stand, and then rang a bell before giving it food. The hungry animal salivated freely in the presence of the food, as would be expected, however Pavlov's dog learned to associate the ringing of the bell with the imminent arrival of the meal and soon could be induced to salivate on hearing the sound alone. Pavlov called the food an unconditional stimulus, because the animal would salivate without fail, or unconditionally, when exposed to it. The ringing of the bell was thus a conditional stimulus, and...