Classical conditioning theory answer

The theory was famously demonstrated by Ivan Pavlov. He observed that dogs naturally salivate (an unconditioned response) at the sight of food (an unconditioned stimulus). By repeatedly pairing the sound of a bell (a neutral stimulus) with the presentation of food, the dogs eventually began to salivate at the sound of the bell alone. In this case, the bell becomes a conditioned stimulus, and the salivation in response to the bell is a conditioned response.