Some dogs have powerful feelings for their balls or chew toys. Never take a bone or toy from a dog's mouth unless you have trained him to drop it and give it to you first. Dogs may get too enthusiastic in these sorts of games and forget you're not a dog. Fetch, Frisbee, hide and seek, agility courses, and flyball are better outlets for your dog's energy. Respect a dog's space.
Dogs naturally defend their territories.
Sticking your hand inside a strange dog's pen or in a car window where a dog is sitting may put him in a defensive situation and he might bite to protect his territory. Leave fighting dogs alone. Most fights end quickly, but it's a good idea to remain quiet and get an adult who can stop the fight with a garden hose or lemon juice in a squirt bottle. Trying to separate or yelling at fighting dogs makes them more excited, and they might turn on you. Observe dog body language.