He showed that the presence of an unconditioned stimulus (food) produces an unconditioned response (saliva of a given amount; varying somewhat between trials and between dogs). After sufficient pairings of the food with another, previously unused stimulus (e.g., light), the conditioned stimulus (light) could bring about a response (conditioned response). The conditioned response was weaker than the unconditioned response (i.e., less saliva) and forgettable (if repeated too often without food being presented).