The soft tennis balls have been made heavier by wrapping them with duct tape and vinyl tape. They are divided into two colors so that you can practice hitting and letting pitches go, for example, "hit the white ones, let the yellow ones go." Because they are hard, they may dent aluminum or similar bats, so we recommend using wooden or Beyond bats. Due to their weight and hardness, they can also be used for batting practice by those who play hardball baseball. Various types of soft tennis balls, such as C balls and J balls, are mixed in.