Barb, this one is VERY hard plastic. I think some vendors call it the "almost indestructible" ball. They LOVE it, and i have had this one for quite some time.
Grammy, most of the time the dogs who play with balls like this have very strong herding instincts. If you watch it, you can see he is "herding" the ball, and when it stops and he has to work to get it moving again is when he barks and gets wound up. Very related to herding/driving behaviors.
