This was demonstrated by Ivan Pavlov with dogs.

An unconditioned stimulus (UCS) normally elicits a specific, unconditioned response (UCR).

If the UCS is associated with a neutral stimulus (NS), the latter may become a conditioned stimulus (CS) and elicit the same response, now called a conditioned response (CR), in the absence of the UCS.

Advertisers often rely on classical conditioning to associate their products with a desirable stimulus.