The bag comes in two pieces.
The first piece is the base of the bag, a reservoir made of thick plastic that can be filled through an opening at the top of the base.
You can fill the base of the bag with either sand or water (or both) depending on what you're looking for in terms of weight and convenience.
The area that will be hit is a second piece, the top of the bag, which is made of foam surrounded by a vinyl canvas. It can be easily attached to the base of the bag.