One traditional way of setting up an alarm is leashing a dog outside your house. Your dog can react two ways when it sees a stranger: first, if the person coming is his master then he will greet by silently wagging his tail and second if the person coming is a stranger it will react by barking. This noise that the dog creates attracts attention to anyone, including the owner that there is a stranger is nearby. The bark of the dog is a form of an ...