Russian scientist, Ivan Pavlov, researched digestion in dogs. In measuring the salivary response to meat powder, he observed that the dogs salivated to the neutral stimuli that was previously associated with it. They began salivating at the sound of the assistants' footsteps coming into the lab. We call the association of the meat power to the footsteps classical conditioning. Pavlov's classical conditioning is defined as a form of learning in which animals come to respond to a previously neutral stimulus that had been paired with another stimulus that elicits an automatic response. This finding is very important in researching relationships between unconditional stimulus and unconditioned response, and the relationships between conditioned response and conditioned stimulus.
A boy at BGSU did a test on his roommate of this study. After hitting the button saying, "that was easy", he would shoot his roommate with an airsoft gun. The roommate soon associated the easy button noise with being shot with the gun. After the boy shot his roommate a couple times, he did a test where he pressed the easy button but did not shoot him. When the easy button was played, the roommate cringed as if he was going to get shot, however, he wasn't actually shot. The easy button was the conditioned stimulus, while the shooting of the gun was the unconditioned stimulus. The unconditioned response was the pain or flinching of the roommate, while the conditioned response was the flinching from hearing the easy button sound.