You may have heard of Ivan Pavlov and his dogs and how he created a conditioned response in the dogs. Basically, what he did was the following:
He brought food to a dog and of course, the dog began to salivate when the food arrived. After a while, he started to ring a bell as the food arrived and of course the dog would salivate. What he then discovered is that if he just rang the bell, the dog would salivate even though there was no food presented – the sound of the bell triggered the same response.