My Jack Russel's favorite activity - playing Ball! - led to many hours playing with tennis balls. He is only 4 years old, but by 3 he already had his canine teeth worn down pretty good from so much playtime. It turns out the TENNIS BALL's outter fabric were acting as an abrasive on his teeth, wearing them down like emory cloth. I had no idea these things were so bad for his teeth. Tennis balls are now off limits, and have been replaced with all-rubber balls that are easier on teeth. Rubber balls also don't soak up as much slobber.