Psychology lesson of the day: In 1901, Ivan Pavlov developed his famous theory of "conditioned reflex." In a paper, the Russian psychologist explained that he had developed the theory through a series of experiments involving his dogs and a dinner bell. Pavlov rang a bell and presented food to the dogs, and the dogs would begin to salivate. When he rang the bell enough times, he found that the dogs would salivate without needing to see any food.

These teenage girls are Pavlov's dogs.

H/T: Reddit