Pavlov classical conditioning transformed the way humans think about learning. With his experiment on dogs, he concluded that animals learn by responding to previously a neural stimulus that was repeated paired with an unconditioned stimulus that caused a unconditioned response. His realized through his dog experiment that when the dogs were given meat power (UCS) the dogs had an automatic reflex to salivate (UCR), which is a response from nature not nurture. The salivating is an automatic response that is not learned but because it is in the nature of dogs to salivate when they are given food. But Pavlov tried something different. He paired up the metronome ticking a neutral response with the meat power (unconditioned stimulus). By constantly paring up the neutral stimulus with the uncontrolled stimulus Pavlov found that the dog went through the process of learning. The dog realized that ticking of the metronome means that the food is about to arrive. So whenever he heard the metronome he stared to drool (unconditioned response), because he associated that noise with the meat powder meaning that he learned. This proved that if an association is made between a neutral stimulus and an uncontrolled stimulus consistently the dog will respond the same way with the neutral stimulus. In this case it was the ticking of the metronome, it now became the controlled stimulus that caused the animal to respond with a conditioned response (CR). The dog learned this through experience not nature.
I found this discovery so interesting that I tried to replicate it with my cat. I got him new treat container that makes a certain noise when you shake it. This noise became the neutral stimulus for the experiment because when I first shook it, it had no response from my cat. Usually when I give my cat a treat (UCS) and his natural instinct in response to the treat is meowing (UCR). The meowing is a response that is not learned but in the cat's nature. Then, I started to repetitively pair up the container noise (neutral stimulus) with a treat (unconditioned stimulus); I did this for a couple of days and observed that my cat had made an association between the container noise and the treat. Gradually, he went through the process of classical conditioning because whenever I shook the container alone (CS) he would run up meow (CR). This is because he has learned from experience not from nature. Since the noise of the container usually got him a treat, he started to associate those two things together. So just hearing the noise of the container makes him meow, which proves Pavlov's discovery. The CR (meowing) now responds to neutral stimulus (container noise) because of the association of the neutral stimulus with the UCS (treat). Pavlov's experiment not only gave an insight on the learning behavior of animals but also the differences between nature and nurture.
The above link is a hilarious version of a modern day classical conditioning in the show The Office. enjoy :)