Pavlov's dogs

Pavlov, accidently, discovered that dogs salivate when they expect food, even if the food is not visible. The act of salivating when there is food is a natural response, a reflex, and it is not learned. However, Pavlov noticed that he could teach the dogs to associate something that has nothing to do with the food, what was named neutral stimulus, with the food, producing in them the same response than the food.
In Pavlov's experiment, he used a bell as a neutral stimulus; each time that he gave the dogs food he rang the bell. After a while, only with the sound of the bell the dogs salivate, so they learnt to associate the bell with the food.

References
McLeod, S. (2015). Pavlov’s Dogs | Simply Psychology. Simplypsychology.org. Retrieved 29 October 2015, from http://www.simplypsychology.org/pavlov.html