Fred's dog was classically conditioned to salivate at the sound of a bell. Fred then repeatedly paired the bell with another stimulus, a whistle. Now whenever he blows the whistle, his dog salivates, even though the whistle has never been paired with food. This example illustrates:

This is an illustration of second order conditioning. Fred's dog associates the bell with food, and therefore salivates. Fred's dog is then trained to associate the whistle with the bell, which is associated with food, and so he salivates. With the removal of the bell and food, the associations are still there. Fred's dog thinks whistle means bell and bell means food, therefore he thinks that whistle means food and he salivates.