If you are taking your dog to a dog park, it is a very good idea to learn how to "read" dog body language. You should learn the difference between a friendly puppy "play bow," which is an invitation to play, and a behavior that could mean that trouble is on the way. Knowing the body language of dogs can give you a "heads up" to potential problems, as well as potential playmates. There are any number of web sites with detailed descriptions of dog body language. The video below is a great example of a variety of common dog body language behaviors. Thanks to Catherine Wyatt for granting permission to link to this video.