[QuickSilver]

I can tell how tired Thor is by how he responds to fetch. He will always, always chase the ball like it's a wounded pigeon. If he's feeling peppy, he will bring the ball back to me, and being the mercenary that he is, he'll get a treat. When he starts to get tired, he'll wander off with the ball and drop it somewhere I can't reach it.

Of course, if I go to pick up the ball, he flips out and wants to chase it again. It can be quite a challenge to sneak the ball back into my purse once it's been out.