Small ballistic nylon outer bag with a matching ballistic nylon filler bag. We designed both our outer bags and our filler bags after using the so-called "duffel-bag" style bag. Ours are better because they give you a better feel of the shifting sand. This is done by allowing the filler bag to hold all the sand the entire length and width of the outer bag. This lets the sand shift as a whole with the outer bag. Small Workout Sandbags are used to do the toughest 3-dimensional moves, and even simple ones such as pushups, pullups, situps, and more. They are great for gals to do anything, or guys to do the really hard moves with. Thank you for choosing Workout Sandbags! You will be glad you did.