Although he won a Nobel prize for his work in explaining the processes of digestion, Ivan Pavlov is best remembered for his discovery of classical conditioning.
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).
If you want to understand why lightning storms are scary, check out Ivan Pavlov.