The origins of tennis can be traced back to a French game played in the 12th century. The modern tennis ball, however, has only been around in its current form since the 1950s. Manufacturing a ball that maintains a constant 15 psi internal pressure while also being covered in yellow felt actually requires a few different disciplines of engineering. Chemical engineers designed a small tablet that when sealed inside the ball, expands into a gas to create the internal pressure. The manufacturing process for these balls is incredibly intricate and mesmerizing. Check out the video above to see how Penn tennis balls are manufactured.