There you are, dog resting quietly next to you, TV at subdued noise level, all is calm. Then there is a sudden knock at the door and all hell breaks loose. Someone knocking can seem like the most exciting thing in the world for certain dogs. They explode into a whirlwind of barking and destruction, not caring who or what gets in their way. If it's the kind of dog who tends be a little over-protective of their home, you may experience them attempting break down the front door themselves, while suddenly adopting the nature of one of the hounds of hell. As for the person who knocked, in all likelihood they'll of quickly left. Making a dog with this kind of behavioural issue quite handy, especially if you're not keen on door-to-door salesmen.