Well, chihuahua's are naturally protective of their home and family, so they are inclined to bark at any moving object. lol. The best way to get them to stop barking without the use of vinegar- which traumatizes them- is simply to teach them the "Stay" command, and tell them to stay before opening the door. If it barks when you open the door, close it again (tell your freinds you are trying to train your dog). tell them to stay, if they bark when you open the door, close it agian. repeat until the dog is no longer barking. Chihuahua's tend to catch on very quickly to firm training. Hope I helped.