Product alert! There are tennis balls made by Tretorn called "Tretorn X" that contain an inert substance which presumably helps with the bounce. However, when the dog pops open the tennis ball while chewing, the substance makes a terrible gooey mess in their mouth and presents an aspiration hazard. I was made aware of this by a client who's dog had a bad experience with just such a tennis ball. Here is a picture of one exposing the material inside.It seems that the material is non-toxic. However, it still presents a potential injury problem. Make your friends aware!