I have a 6-year old terrier mix dog who has one truly annoying trait. When he plays with other dogs (and he definitely is playing) he barks continuously. He does this (barking) with all dogs he's playing with, and no matter where we are... inside, outside, at the dog park. There's no aggression beyond the barking. He alert barks at other times too, but I can quite him pretty quickly in general. I have recently moved into an apartment complex, so while in the past I have simply tolerated the barking during play, I really don't think that's appropriate to my living situation. My question is this... how do I indicate to him that the barking is a no-no, without stopping him from the play with the other dog?