A grinder is a tool for breaking large pieces of plant matter into a fine, malleable, powder-like consistency. Grinding plant matter makes it easier to load into a bowl, allows the material to burn more easily, and exposes more of its surface area to the airflow. Grinders can have anywhere between one and three chambers. The most essential chamber is the upper chamber, where the grinding actually occurs. Metal or plastic teeth lock together with a magnet, and the topmost piece of the assembly is twisted repeatedly to crush the plant matter inside. The middle chamber of a grinder is typically used for storage. Holes in the bottom of the upper chamber allow ground plant matter to fall into the storage chamber, where it can be retrieved more easily. The lowermost chamber is covered by a fine mesh screen, which filters out all but the tiniest plant particulate, known as kief. The bottom of a three chamber grinder can be unscrewed to access pure kief that has accumulated there. GRAV® grinders have three chambers made from CNC aluminum, and the middle storage chamber features polycarbonate windows that allow a snapshot of the grinder's contents.