Answer: :)
Ivan Pavlov, a Russian psychologist, conducted a psychological study during the 1890s regarding the interrelationship between stimuli and response and the ways in which each could be conditioned to the other. Out of the various research methods available, Pavlov chose to conduct an experiment in order to support his concept of classical conditioning. The technique of classical conditioning went on to become an important part of the behavioral branch of psychology.
In one of the most famous experiments in the field of psychology, Pavlov observed that his dog responded to food, which was accompanied by the sound of a bell, by salivating. The psychologist came to realize that the dog had learned this response (salivation) based on the appearance of food. In this case, the sound of the bell was an independent and neutral stimulus. Pavlov decided to pair the sound of the bell with the availability of food (the unconditioned stimulus). According to Pavlov, the unconditioned stimulus was responsible for triggering an unconditioned response from the dog (i.e., salivation).
As part of the conditioning experiment, Pavlov combined the sound of the bell (neutral stimulus) with the availability of food (unconditioned stimulus). When the dog started making an association between the two stimuli, it began salivating at the sound of the bell irrespective of the availability of food. At the end of this experiment, Pavlov termed the sound of the bell as a conditioned stimulus (formerly a neutral stimulus), and the salivation as a conditioned response (formerly an unconditioned response).