Ivan Pavlov won a Nobel Prize for research on digestion, not psychology. In the 1890s he was measuring how much dogs salivated in response to food, using a precise surgical procedure to collect and quantify their saliva — and the experiment that made him famous was, in his own research, an annoying complication: his dogs started salivating before the food ever arrived, just at the sound of the approaching footsteps or equipment that usually preceded it.
From a data problem to the discovery itself
Rather than treating the early salivation as noise to be eliminated, Pavlov turned it into the actual object of study. He began deliberately pairing a neutral stimulus — most famously described as a bell, though his original experiments used various sounds and signals — with the arrival of food, over repeated trials. Before conditioning, the bell alone produced no salivation; food alone reliably did. After enough repeated pairings, the dogs began salivating at the sound of the bell alone, even with no food present at all. Pavlov named the food the "unconditioned stimulus" (it produces the response naturally, with no learning required), the salivation to food the "unconditioned response," and the learned salivation to the bell the "conditioned response" — a completely new association the brain had built purely through repeated pairing.
Why the pairing has to be consistent to stick — and can be undone
Pavlov's later experiments mapped out the rules governing how this kind of learning behaves: the pairing generally needs to be reasonably consistent and reasonably close in time to form reliably, and once formed, the learned response can also be weakened or eliminated — a process called extinction — by repeatedly presenting the bell without the food, until the dog stops expecting food and the salivation to the bell fades. He also found the learned response would generalise to similar stimuli (a similar-sounding tone) and could be trained to discriminate between stimuli (respond to one specific tone but not a slightly different one), which turned what started as a mildly annoying wrinkle in a digestion experiment into a foundational, systematic account of how associative learning actually works.
What we're still unsure about
Classical conditioning's core mechanics are thoroughly replicated and form a foundational part of behavioural psychology, but the field has moved well past the idea that this kind of automatic, repetition-based pairing explains most of complex human and animal learning. Later research — including on cognitive expectations and predictive processing in the brain — has shown classical conditioning involves more than pure repetition; the brain appears to track the actual predictive reliability of a signal, not just how often two things happened together, and exactly how much genuine "prediction" versus simple association is happening even in Pavlov's original dogs remains a live question in learning theory and neuroscience.
This sits inside Classical Conditioning (Pavlov), one of seven topics in Behavioral Psychology, one of four domains in Psychology, one of seventeen subjects the app can quiz you on.