In the early 1900s, Russian Physiologist Ivan Pavlov, measured the saliva that dogs produced when he brought them food, as their bodies prepared to digest the food. Then for a while he rang a bell at the same time he brought the food. Then he rang the bell but did not bring food. The dogs produced saliva exactly as if he had brought them food. In other words an unconscious physiological response was produced to stimulus that would not have normally produced that response.
This is called a conditioned response and usually happens to us more powerfully with bad or fearful things. These responses can be very specific. One lady got terrified when she heard drinking glasses clink together because it reminded her of the start of an earthquake.