Penn QST 36 Foam 2 Pack The Penn QST 36 Foam 2 Pack features a quality foam training ball with an oversized diameter that's perfect for learning the basics of the game of tennis. These balls are made with a high-density lightweight foam to provide a slightly modified bounce and are perfect for use on indoor courts. Featured in Red for 36' tennis court sizes. Ideal for use with players ages 8 and under. Approved by USTA and ITF.