My brother has a Aussy. Hottest day of summer that dog will fetch a ball/frisbee endlessly, and you actually have to make the dog stop, drink water, and rest in the shade. Left to its own vices, I bet it'd play fetch until getting heat stroke.

Decided to give the girls a few more throws. I did the old quarter back drill of seven step drop and fifteen yard down and out pass with a Frisbee. Those girls never dropped one and I'm (have to) going to lay down for a bit

Side note: The border and aussy get their butts kicked by our two kitties, they just can't leave them alone. They all sleep together every night after thing calms down. Really cute. I think the cats know the dogs have their six.