"Conditioned Reflexes: Drooling at the Bell"
Russian physiologist Ivan Pavlov accidentally discovered that his dogs started salivating not just at food, but at the footsteps of the lab assistant. He formalized this as Classical Conditioning. He rang a bell (neutral stimulus) before giving food (unconditioned stimulus). Eventually, the dogs drooled at the sound of the bell alone.
This showed that behaviors can be learned through association. It explains many human behaviors, from phobias (associating a dog with a bite) to advertising (associating a product with happiness). We are constantly being conditioned by our environment.