Classical conditioning demonstrate learning by association:

This type of learning was first investigated by Ivan P. Pavlov. He was primarily interested in the physiology of digestion. During his studies he noticed that dogs, on whom he was doing his experiments, started secreting saliva as soon as they saw the empty plate in which food was served. Saliva secretion is a reflexive response to food or something in the mouth. Pavlov designed an experiment to understand this process in detail. He again used dogs. In the first phase,, a dog was placed in a box and harnessed. The dog was left in the box for some time. This was repeated a number of time on different days. In the meantime, a simple surgery was conducted, and one end of a tube was inserted in the dog’s jaws and the other end of the tube was put in a measuring glass.

In the second phase of the experiment, the dog was kept hungry and placed in a harness with one end of the tube ending in the jaw and the other end in the glass jar. A bell was sounded and immediately thereafter food (meat powder) was served to the dog. The dog was allowed to eat it. For the next few days, everytime the meat powder was presented, it was preceded by the sound of a bell. After a number of such trials, a test trail was introduced in which everything was the same as the previous trials except that no food followed sounding of the bell. The dog still salivated to the sound of the bell, expecting presentation of the meat powder as the bell had come to be connected with it. This association between the bell and food resulted in acquisition of a new response by the dog. i.e., salivation to the sound of the bell.

Food is thus an Unconditioned Stimulus (US) and salivation which follows it, an Unconditioned Response (UR). After conditioning, salivation started to occur in the presence of the sound of the bell. The bell becomes a Conditioned Stimulus (CS) and saliva secretion a Conditioned Response (CR). This kind of conditioning is called Classical conditioning. The learning situation in classical conditioning is one of S-S learning in which one stimulus (e.g., sound of bell) becomes a signal of another stimulus (e.g., food). Here one stimulus signifies the possible occurrence of another stimulus.