Why do dogs growl and show their teeth?

Typically if a dog is showing it's teeth it is telling you to back off or to stop what you are doing. It's warning sign. It does not always mean they are about to bite but they could be getting close to that point.

Emmy the awesome.......

Well it just matters on the situation, if you are trying to smack them from them getting something it means that they are protecting themselves, and if you try to pick them up when not the best time they'll do it becuase they were comfortable and wanna be left alone, so ya just depends on the situation like I siad.