Rescorla–Wagner (R-W) Model Of Classical Conditioning
The Rescorla–Wagner (R-W) model of classical conditioning was created in 1972 by researchers Robert A. Rescorla of the University of Pennsylvania and Allan R. Wagner of Yale University. This model was intended to illustrate how classical conditioning works: the unconditioned stimulus (US) starts as a surprise reaction to a stimulus but then becomes routine as it becomes a conditioned reaction to an expected conditioned stimulus (CS).
The greater the surprise the stronger the learned association. One example of this is the act of teaching a puppy to sit. First you say "sit" to the dog (unconditioned stimulus) and push its hindquarters into the sitting position. You then give the puppy a food reward. After a few repeats of this the puppy will associate the word 'sit' with sitting down and getting a treat. Shortly you will not have to physically position the puppy, it will understand the combination of food and "sit" and do it voluntarily.