Wool, of course!
A baseball has three basic parts: the round cushioned cork pill at its core, the wool and poly/cotton windings in its midsection, and the cowhide covering.
Wool was selected as the primary material for the baseball’s windings because its natural resiliency and “memory” allow it to compress when pressure is applied, then rapidly return to its original shape. This property makes it possible for the baseball to retain its perfect roundness despite being hit repeatedly during a game. A poly/cotton blend was selected for the outer winding to provide added strength and reduce the risk of tears when the ball’s cowhide cover is applied.
The wool yarn is wound so tightly that it has the appearance of thread when a baseball is dissected. Three layers of wool are wound around the baseball: the first, 121 yards of four-ply gray yarn; the second, 45 yards of three-ply white; and the third, 53 yards of three-ply gray.
A layer of 150 yards of fine poly/cotton finishing yarn is wrapped around the ball to protect the wool yarn and hold it in place. The wound ball is then trimmed of any excess fabric and prepared for the application of the external cowhide covering by being dipped in an adhesive solution.
The cover of an official baseball is made of white Number One Grade, alum-tanned full-grained cowhide, and it must be stitched together with 88 inches of waxed red thread.