made of 1000D military grade Cordura with outer velcro closure, 2 zipper linings( outside zipper is horizontal, and the inside zipper is vertical) for added security and durability, and one industry grade robust thick plastic bag inside for containing sand. Can fill up to 100KG of sand (could be heavier if fill lead balls). For strongman, functional, and crossfit training. Not for punching. You can buy sand from hardware store or take from the beach. Please do not fill with sharp objects such as rocks and shell as they may gradually protrude the material.