By just two weeks old, puppies are typically happily wrestling with their litter mates. While it might just look like fur balls having fun, this early play is critical to a puppy's social development. Dogs playing together from such a young age teaches them communication and self-control. After all, if you bite one of your litter mates too hard he won't play with you anymore.

As puppies age and grow, they don't lose their playful spirit. Give your dog the opportunity to make four-legged friends, but be vigilant. It's up to you to supervise and make sure your pup is enjoying good-natured play and isn't getting too aggressive with other dogs.

Ready for Fun

Here are a few signals dogs use to show other pups they're ready to play:

• Dropping into a "play bow": You might see your dog put his front legs on the ground and his back end in the air as he faces his pal. Really exuberant dogs might even slap their front legs on the ground to show they're ready to romp.
• Taking turns: Dogs playing together will sometimes play a form of tag, where they take turns chasing each other.
• A smile: You know when your dog is happy. You might actually see him looking as if he's grinning as he races around a dog park with a friend.
• Exaggerated growling or barking: Puppies "play growl" and your dog might not have outgrown this puppy behavior. It could sound scary, but if other behaviors show you that your dog and his friend are just having fun then don't be alarmed.
• Play Biting: This is typically one of the hardest signs for pet parents to accept because we associate biting as a negative outside of eating, but this is simply what dogs do. It's not uncommon for one dog to submit and lie on his back, while the other dog nips at his ears or nose. Both dogs may bear their teeth, but as long as there isn't aggressive growling, yelps or whimpers your dogs are probably just engaging in playful behavior. If you notice one dog isn't feeling it and looks like he'd rather be left alone, it might be best to separate them for a time. This can be common with puppies trying to engage in play with an older dog that just wants to take a nap.

Crossing the Line

When does a line get crossed between play fighting and a dog actually exhibiting aggressive behavior?

Signs of dog aggression include raised hackles, stiffness, snapping, or lunging. If either dog shows aggression, separate them immediately. But be careful: Never get between two fighting dogs.

Dogs can also become territorial, whether it's toward a place, food, a toy or a person. If you notice one of your dogs becoming territorial over something any time another dog comes around it, it is best to separate them before aggressive behaviors begin. Then, work with an obedience trainer to try and get to the bottom of this behavior and train it out of him. This can often happen when introducing a new dog into the house with an older one. The older one is not used to having to share his toys or your love, so it might take a little extra training on your end to get him used to the idea of sharing his home.

If you have a dog that is aggressive, it simply might be best for you to avoid situations that may cause your dog to fight. But if you have had a dog that has shown signs of aggression in the past, you should be on alert for his whole life. A relapse can occur at any point. Consult your veterinarian if this behavior is regular. You might also need to consult a behavior trainer that can train your dog the right and wrong ways to act if you are struggling to train him to play nice.