Our year-old German Shepherd, Cushla, is crazy – I mean nuts — about tennis balls. They are just about her favorite thing on earth, except food and rawhide chews.

When she thinks we're going to throw the fuzzy yellow spheres for her, her leaps know no bounds. She jumps up in the air, as if her legs had springs, just to show how excited she is at the prospect of chasing them. We have to keep them where she can't get at them in the house, or she would chew them to pieces.

Mind you, standing next to the dry sink where the balls are kept is enough to make the avid pup think a fetch session is imminent and start levitating. Energy-efficient she isn't.

And if one of us picks up the Chuckit! launcher we use to throw the balls, her frenzy gets even wilder. Now she's certain we're going to take her out to play. Actually, her enthusiasm is sometimes enough to inspire an impromptu play session.

I discovered Cushla's affinity for chasing spherical prey by chance. When she was a puppy, we would throw Frisbees for her to chase, because that's what our older Shepherd, Dominic, likes. She would run after the Frisbee, then stop where it landed, look at it for a minute, and trot back, hoping we'd throw something else. She wouldn't bring back the Frisbee more than once in eight or 10 throws.

One day after I'd gotten frustrated again trying to play fetch the Frisbee with Cushla, I decided to try a ball instead. Bam! She shot after that thing and brought it right back to me. And the next one, and the next. It was like she was just waiting for us to stop throwing the hard, flat discs and get out the nice, round, bouncy, chewable balls.

Now that we have the Chuckit!, playing fetch is a way to exercise our highly energetic girl when it is too cold or we don't have time to take her for a walk/run. That's a walk for us, a run for her, as she doubles back periodically to check on us, then bounds ahead to investigate or play with Dominic.

Since we lack the best thing to make a puppy tired – an active child – we make do with throwing balls and taking her for walks. Dominic is a big help as well. At 8, he's not as naturally energetic as Cushla, but she inspires him to wrestle and play chase, which is good for both of them. And exercising her is also exercise for us – no wonder they say dogs make you live longer.

I can take the launcher and a couple of tennis balls out on the deck and throw them down the driveway. Cushla tears after them and trots around me in a proud circle with her prize before putting it down to get another ball thrown for her. Or I can throw the ball away from the driveway, up the hill behind our house. Not bothered in the least by a little hill, she chases the ball down and brings it back.

Throwing tennis balls this winter, we lost a few in the deep snow to either side of our driveway. But, like flowers in spring, they were revealed by warmer weather. I went outside the other day and discovered two Cushla had found and brought up to the deck. The clever girl knew just where they belonged.

Speaking of clever, she also learned to play with her loudest squeaky toy just as we're sitting down to watch TV in order to get us to give her a rawhide chew.

Crazy — like a fox.