Yorkshire Terriers are dumb, difficult to house break, and tend to be territorial despite the limitations of their size. They often attack strangers and larger animals, disregarding their own size disadvantage. These behaviors are, however, encouraged by their owners because they see it as being "cute".
These dogs are probably the best example of artificial selection. They have weak bone structure which causes them to be unable to survive short falls. They also are known to suffer from things like seizure, liver shunt, and collapsed trachea.
Yorkshire Terrier Owner: It's dominating. Isn't it cute?
Normal Dog Owner: Well it's going to dominate its way into my dog's stomach if it isn't careful.
Yorkshire Terrier Owner: Help, my dog broke every bone in its body!
Vet: What the hell happened?
Yorkshire Terrier Owner: It jumped off my foot stool!
Yorkies have horrible tempers; they will frequently dig their needle-sharp teeth into people for no apparent reason, such as when they sit next to one of these dogs within six feet of them on a different couch. They pick fights with dogs six times their size, which is amusing, especially when a mastiff or something rips it to pieces. Yorkies become loyal to one master, and they hate everyone else and will bite complete strangers. I've seen it happen, and it happened to me once too.
If you have a yorkie in your house do not try to talk the owner into seeing how much of a little terror the dog is. Simply take it outside, tie it up, pull out a shotgun (I would reccommend a 16 gauge or bigger for maximum effect) and blow the little creature all over the pavement, then follow up by burning the remains just to be sure.
"OH MY GOD!!!!!"
"Oh pipe down, you crusty snatch faced mother fucker, the world's a better place now."
They are bright, active, fiesty, athletic, loving and portable little dogs.