Classical conditioning is a learning procedure in which a biologically potent stimulus (for example food) is paired with a previously neutral stimulus (for example, a bell).
Ivan Pavlov, a Russian physiologist, is widely praised for his contributions to the learning procedure. During the 1890s, he was researching salivation in dog's responses to being fed. He used all sorts of dogs, including many mutts.
In his experiment, he inserted a small test tube into the cheek of each dog to measure saliva when the dogs were fed.
He came across classical conditioning unintentionally during his research. While measuring the salivation rates, he found out that they would produce saliva when they heard or smelt food in anticipation of feeding.
Nowadays, classical conditioning learning procedure is one of the fundamental techniques for learning and training.