Who's the Master?

Today I watched an extraordinary dog. He was playing catch (OK fetching the ball his owner was throwing). These balls were going fast and far and the dog was really working to either catch them in the air or on the first bounce. Sounds sort of boring but the fascinating part was the dog. He was fully in control of this game. Each time he caught the ball he decided when to give the ball back for the next throw. If after a couple of catches he was breathing hard, he would stop and sit about 10 yards away and rest. When he was ready, he picked up the ball walked over to his "master?" and would hand it over for the next throw. I watched him for about 15 minutes and found it fascinating.

The picture isn't anything special, with a slow lens that wasn't long enough, in flat but fading light. It certainly isn't my best of the day but the whole interaction between the "masters" was a delight to watch.