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How can a warning light or buzzer become a signal that an animal avoids before receiving a shock?

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A fluffy gray cat clings to the trunk of a tree, looking upward as it climbs to escape. — Avoidance behaviour Cat fleeing by upward climbing.

A warning light or buzzer becomes a signal that an animal avoids through a two-stage learning process involving classical conditioning.1

First, the animal is subjected to pairings of the neutral stimulus (the light or buzzer) with an unavoidable electric shock. Through this association, the animal learns to fear the signal itself, as the signal becomes a predictor of the painful stimulus.1

In the second stage, the animal is given the opportunity to perform an action—such as running away or turning a paddle wheel—to terminate the warning signal. Because the signal has become a source of learned fear, the act of stopping it serves as a reward. By reducing this internal state of fear, the animal is motivated to perform the avoidance response whenever the signal appears, even if the shock is no longer presented. This allows the animal to successfully avoid the adverse stimuli before the shock occurs.12