Pavlov trained dogs to salivate at a sound alone, no food involved

Ivan Pavlov noticed his lab dogs drooled at the mere footsteps of the assistant who fed them, not just at the food itself. By repeatedly pairing a neutral sound with feeding, he showed dogs learn to salivate at the sound alone, the founding demonstration of classical conditioning, still the backbone of how psychology explains learned associations.

— Ivan Pavlov, Conditioned Reflexes — Lectures 1897-1903; published 1927
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