As the dogs became familiar with the laboratory routine, Pavlov found that they began to salivate before he presented the food. The dogs had learned to anticipate the food, so he constructed screens to obstruct their view. Moreover, before presenting the food, he introduced an unrelated stimulus, a ticking metronome. At first, the dog drooled only to the meat. However, after a number of trials, the sound of the metronome alone triggered salivation. Pavlov called this new response the conditioned reflex. He found that his dogs could be conditioned to produce saliva to a variety of stimuli. Pavlov believed he had discovered how animals learn.