How did a metronome become a conditioned stimulus in Ivan Pavlov’s experiment?

In Ivan Pavlov’s experiments, a metronome became a conditioned stimulus through a process of associative learning known as classical conditioning.

Pavlov utilized a standard procedure where a hungry dog was restrained and presented with an unconditional stimulus—food—which naturally elicited an unconditional response of salivation. By consistently pairing the ticking of a metronome with the delivery of food, the metronome—initially a neutral stimulus—became a predictor of the meal.12

After several trials, the dog learned to associate the sound of the metronome with the arrival of food. Consequently, the metronome alone became capable of eliciting salivation, which is then termed a conditional response. This learned association is reinforced by the repeated temporal pairing of the metronome sound immediately preceding the food. If the metronome is presented repeatedly without the food, the conditioned response will eventually undergo extinction.1