Were not the only ones who love playing sports. Flyball is a sport for dogs, and they have regulations for their balls that are used in the sport.
Section 1.2 – Balls
Each club shall have a supply of round balls subject to the size, safety, and comfort of the dogs. Approved balls, to be retrieved from the box, may be any color, must bounce when dropped onto a hard surface, must roll, and must perform as required per Chapter 1 Section 1.1(d). Balls may not contain a noise maker, e.g. bells or squeakers.
Flyball Rulebook – http://www.flyball.org/rules/rulebook.pdf
Photo – http://www.lightningstrikesflyballteam.co.uk