A small thing to chase.
A ball built for the rougher side of play.
Play isn't always careful - there's a batting, a gripping, a mouth working at something until it gives. Most toys aren't built for that kind of attention, and don't last through a single afternoon of it. The sisal ball is: dense enough to survive being caught again and again, coarse enough to give claws and teeth something honest to work against, small enough to end up forgotten under the couch until the next burst of energy finds it. A humble object, built for the part of a cat's day that isn't gentle.