Classical conditioning demonstrates learning by association through the process where an organism comes to associate different stimuli. This type of learning was first investigated by Ivan P. Pavlov during his studies on digestion in dogs.

Pavlov noticed that dogs started secreting saliva as soon as they saw the empty plate in which food was served. In classical conditioning, a neutral stimulus (CS) that signals an unconditioned stimulus (US) begins to produce a response that anticipates and prepares the organism for the US. This response is called the conditioned response (CR).

For example, a small child catches an inflated balloon which bursts in her hands making a loud noise. The child naturally becomes afraid. Now the next time she is made to hold a balloon, it becomes a signal for noise and elicits a fear response. This happens because of contiguous presentation of balloon as a conditioned stimulus (CS) and loud noise as an unconditioned stimulus (US).

Another example is when after finishing lunch, seeing a sweet dish signals its taste in the mouth and triggers saliva secretion. This is a conditioned response (CR). The organism learns to associate the sight of the sweet dish with its taste.

Thus, classical conditioning clearly shows how learning occurs through association between stimuli, where a previously neutral stimulus becomes capable of eliciting a response through repeated pairing with another stimulus.