A dog stands motionless in the middle of a room, immobilized by a leather harness. The room is very quiet, all outside sound blocked by one-foot-thick concrete walls. A bell rings, and the dog turns toward the bell but otherwise shows little reaction. Five seconds later, the dog is presented food powder through a long rubber tube. The silence returns. Ten minutes pass; the bell sounds again and, as before, is followed by food. Ten more minutes pass. Again the bell sounds, but this time the dog begins to move restlessly in its harness, saliva dripping from its mouth. As the trials continue, the dog appears increasingly excited when the bell sounds, with more and more saliva flowing into a tube that has been surgically implanted in the dog's mouth. The saliva flows through the tube into an adjoining room where technicians record the number of drops.

When word of this experiment reached other scientists, the news was greeted with tremendous excitement. Within a few years, virtually every psychologist in the world knew the experimenter, Ivan Petrovich Pavlov. Within a few decades, his research had become perhaps the best known in the history of science, ranking with the legendary fall of an apple onto Isaac Newton's head.