In the twentieth century, formalized dog training originated in military and police applications, and the methods used largely reflected the military approach to training humans. In the middle and late part of the century, however, more research into operant conditioning and positive reinforcement occurred as wild animal shows became more popular. Aquatic mammal trainers used clickers (a small box that makes a loud click when pushed on) to "mark" desired behavior, giving food as a reward. The change in training methods spread gradually into the world of dog training. Today many dog trainers rely heavily on positive reinforcement to teach new behaviors.