Training a dog to be a watchdog often reduces unexpected results. Some dogs quickly learn the difference between unwanted people and friends. This is because their masters welcome friends into their houses. However, some dogs will always attack the postman who comes to deliver letters. One explanation for this behavior is that, although the postman comes to the house often, he never enters the house. Therefore, the dog thinks the postman is someone not wanted, but keeps coming back anyway. What is the main idea?