The water bag is a donut-shaped weight that can be filled using water. When full, the weight adds 16 lbs to the cross base (this weight can be placed on top of the square base as well). Though designed to be filled with water, the bag could be filled with sand Do not mix sand and water, as this will create an unwanted and bothersome mess. With that in mind, we recommend only using water to inflate the bag, as this will ensure ease in repeatedly filling and emptying for on-the-go needs.