Ivan Pavlov, a famous psychologist and physiologist, lived from 1849 to 1936 in Russia. Pavlov theorized that he could make a dog drool by simply ringing a bell. He was able to do this by employing the technique of classical conditioning. Pavlov repeatedly rang a bell before he would feed his dogs. By doing this Pavlov's dogs were conditioned to drool every time they heard the bell.
Though this discovery may seem quite simple it is one of the most important discoveries in the field of psychology. In fact, this discovery formed the basis of what we now know as behavioral psychology. Without this discovery we wouldn't have ways treat various psychological disorders such as anxiety and panic attacks. We also would have a much tougher time training our animals.
There are countless cases of people using classical conditioning to train their animals. An example of this is that when fish hear footsteps on a dock they come to the surface and wait for food. The reason they do this is that they are so accustomed to getting food when they hear footsteps that they automatically surface even if there isn't any food.