A CO₂ cartridge is a small, sealed metal cylinder (usually steel or aluminum) filled with compressed carbon dioxide gas. Cyclists commonly use them with a CO₂ inflator head to rapidly inflate flat tires while out on the ride. They come in different weights (e.g. 16 g, 20 g, 25 g, 28 g) and with different threads (often ⅛" NPT or similar). Because the gas is under pressure, one must handle them carefully and use a proper inflator to control the release.