I mentioned previously that our dog is aggressive. We have to walk her with a Gentle Leader collar. We don't use a muzzle because she's never attacked anyone outright, but she certainly makes a show of scaring them off. The Gentle Leader collar is a great alternative to a muzzle because she can still open her mouth, but when she pulls it won't choke her and instead forces her head downwards.
The problem is that while we are aware of her personality and behaviors other people are not. I was surprised at how many parents didn't know how to read her body language. The children certainly don't but the parents don't see her as threatening because she is a lab and they are known to be more congenial as a breed.
The misinterpretation comes in the form of her wagging tail. She wags it as a sign to us her "pack members" that there is danger afoot. The majority of people automatically associate a wagging tail as a friendly greeting. Let me assure you that this is not always the case.
Depending on the angle of the tail and the rest of the body, the wagging tail is a sign of warning and aggression. She knows that there is a potential threat. In her case, a threat is any other moving object except strangely construction workers and wild bunnies. She absolutely detests small children and cyclists. The cyclists "sneak" up on her, and I think she distrusts children for their noise and that they approach her because they want to pet her. At least they usually ask permission and we always say NO! She is much too worked up and sometimes foaming at the mouth with excitement. I don't think she would bite anyone because she does try to hide behind us if they get too close, but she could hurt them by jumping up on them.
So, here is a quick explanation. If the tail is raised up from the body and not level with the spine, then the dog is not being friendly or calm. They are feeling either aggressive or trying to assert dominance. If the tail is level with the spine or lower, then they are calm and submissive and you can probably pet them safely. Of course it's always a good idea to ask the owner's permission.
That is probably the most useful thing to know about dog body language.
The one exception to the tail position is when the dog is in "play position" which also includes a lowering of the front half of their body with the tail raised in the back. They'll usually jump up and down in this position a few times to show that they are eager to play with you. Rosie does this a lot and she loves it when you mimic the motion back to her. :D
Certainly this is one thing to tell your kids. We live near a park and there are always children wanting to pet her but they don't seem to understand why it's not a good idea. By pointing this behavior out to them, they can avoid approaching dogs that are not friendly and could potentially harm them. From personal experience, I can tell you that dog bites are not fun. Not from any of our dogs, but a neighbor's dog that approached me because it thought that I was a threat.
Remember that looking them in the eyes is a sign of dominance or aggression. Don't approach aggressive dogs. Always present a new dog with the back of your closed fist to sniff before petting them. At least then if they do try to bite or nip you they won't get your finger.