When we see flashing lights or hear a siren behind us while driving, we may reflexively tense up and our heart rate may increase. We have been conditioned to associate the sound of sirens with the unpleasant and stressful experience of getting a ticket. This is classical conditioning. Both animals and people can learn to relate a pair of events and respond to the first in anticipation of the second. This type of learning is passive and involuntary; it occurs without the learner doing anything and often without awareness.
