Patented Fill & Drill Tennis Trainer fills with water to become heavier than any other tennis trainer on the market, but is light and portable when empty. Great teaching tool for beginners and intermediate players. Patented Fill & Drill Tennis Trainer fills with water to become heavier than concrete based tennis trainers. very light and portable when empty, can be stored in players bags and taken along. Elastic rubber cord winds around base, ball fits securely into base pocket. Great teaching tool for beginners and intermediate players.