Color Apple Green Fair Pre-owned condition Size M (Both items) Some pulling in the sweatshirt only The sweatshirt has a crease mark on the front. What is Pilling on Fabric? Those tiny lint balls on your sweaters and bed sheets are actually small knots of broken fibers. These knots, called pills, can form on any fabric, but often appear when the fabric is a blend of different fibers or has a loose weave, like knit fabrics.