Cutaway shows hollow core.

Markwort Safe-T-Ball® 9" Baseball

Kids play without the fear of getting hurt from a heavier ball.
It is the same size as a standard baseball, but weighs only
3.5 oz. (A traditional size baseball weighs 5 oz.) If a child gets
hit, the patented rubber hollow core absorbs some of the force of impact to lower the risk of injury.

Safe-T-Balls® do not prevent all injuries. They can help reduce
the sting from impact, and that increases confidence for kids
who fear getting hurt by a standard baseball or softball.