title: Tennis ball
context: Tennis balls are filled with air and are surfaced by a uniform felt-covered rubber compound. The felt delays flow separation in the boundary layer which reduces aerodynamic drag and gives the ball better flight properties.[3][4] Often the balls will have a number on them in addition to the brand name. This helps distinguish one set of balls from another of the same brand on an adjacent court.[5]
question: What is a tennis ball made out of?
answers: filled with air and are surfaced by a uniform felt-covered rubber compound