Classical conditioning belongs to the school of learning theories termed behaviorism. Ivan Petrovich Pavlov undertook the classical demonstration of conditioning in the early 1900s. He showed how dogs could learn to associate the ringing of the bell with the presentation of food, and after a while would salivate at the sound of the bell alone.
He measured the amount of saliva produced by a dog and showed that this increased as the animal learned to associate the sound of the bell with the presentation of food. The dog became conditioned to respond to the sound of the bell. Such learning is widespread among animals.
Pavlov’s experiment involved positive conditioning, but negative conditioning can also occur. For example, a young bird quickly learns to associate the black-and-orange markings of the cinnabar moth’s caterpillars with their unpleasant taste and to avoid eating such caterpillars in the future.