The classic examples of classical conditioning are Pavlov's dogs and Watson's Little Albert. This is not surprising. Every farm boy for thousands of years has realized, of course, that animals become excited when they hear the sounds that indicate they are about to be fed. But Pavlov carefully observed and measured one small part of the process. He paired a sound, a tone, with feeding his dogs so that the tone occurred several times right before and during the feeding.
Classical conditioning sounds simple.
Actually, there are many complexities. That's why Pavlov persisted for 30 years. He discovered many of the basic learning processes, such as the necessary timing when pairing the conditioned stimulus with the unconditioned stimulus, inhibition, extinction, generalization, discrimination, higher order conditioning, and others. All still described in Introductory Psychology textbooks today. Pavlov concluded that all psychopathology was learned via classical conditioning.