Discovered by Ivan Pavlov in the 1890s.
Pavlov noticed dogs drooled not just at food, but at the sound of the footsteps bringing the food.
Experiment:
1. Ring bell -> Give food (Repeated)
2. Ring bell -> Dog drools
This is "Classical Conditioning." A neutral stimulus (bell) becomes associated with a potent stimulus (food).
It explains many of our automatic reactions.
Behavioral
Pavlov's Dog
Why dogs drool at a bell