Ivan Pavlov was a physiologist studying the digestive system in dogs, when he noticed that the dogs would begin to drool (or "salivate") when ordinary things happened around the time when food was coming. Pavlov found that he could train the dogs to drool following a particular cue, like the sound of a bell, by presenting the cue before giving the dogs their food. Over time, the dogs would begin to drool when they heard the cue rather than when they received their food.
This process is not limited to dogs. For example, if you go to an ice cream shop a lot and it smells like strawberries, eventually you might come to form a mental link between the ice cream and the smell of strawberries. The smell of strawberries becomes a cue that could cause you to start salivating at the thought of ice cream. This process of forming mental links between environmental cues and something that is highly motivating, such as a food you like, is called classical conditioning.
Scientist have found that classical conditioning also happens in drug addiction.

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