Pavlov inserted a test tube containing a powder made from meat into each dog’s cheek to measure their levels of saliva when they were fed.
Pavlov predicted the dogs would salivate in response to the food placed in front of them, but he noticed that his dogs would begin to salivate whenever they heard the footsteps of his assistant who was bringing them the food.
Pavlov discovered that any object or event which the dogs learned to associate with food (such as the lab assistant) would trigger the same response.
Food is an unconditioned stimulus meaning a response takes place without any prior learning.
Pavlov used a metronome as his neutral stimulus. By itself the metronome did not elecit a response from the dogs.
Pavlov began the conditioning procedure, whereby the clicking metronome was introduced just before he gave food to his dogs. After a number of repeats (trials) of this procedure he presented the metronome on its own.