The Tourna Fill & Drill Tennis Trainer allows for individual practice that can be done anywhere. When filled with water or sand, the base becomes heavier than any other Tennis Trainer on the market. Some tennis trainers are not heavy enough to not move on the ground and some tennis trainers are too hard to fill up. Tourna's Fill-n-Drill is the easiest tennis trainer to fill up and its the heaviest on the market. Very light when empty, it can be stored in players bags and taken along. Elastic rubber cord winds around base and ball fits securely into base pocket. Great teaching tool for beginners and intermediate players.