Pavlov became interested in studying stimulus when he saw that the dogs in the labs drooled before they were fed their food. It turned out that the dogs were reacting to the lab coats. Every time the dogs were served food, the person who served the food was wearing a lab coat. Therefore, the dogs reacted as if food was on its way whenever they saw a lab coat.
In a series of experiments, Pavlov then tried to figure out how these occurences were linked. For example, he rang a bell when the dogs were fed. If the bell was sounded in close association with their meal, the dogs learnt to associate the sound of the bell with food. After a while, at the mere sound of the bell, they responded by drooling.