a long and narrow metal draw tube that can be found in every modern beer keg. It reaches from the neck valve to within a half inch (1.25 cm) of the bottom of the keg. It allows gas under pressure to enter the keg and drive the beer to the tap. The beer flows up the spear and out of the coupler to the tap by way of a ball valve inside the neck body on the keg head (the top of the upright keg). When the tap is opened to pour the beer, the pressure inside the keg propels the beer. External gas, usually carbon dioxide, is regulated for a particular pressure level and replaces the depleted beer, maintaining a constant keg pressure until the keg is emptied. The spear is usually held in place by a clip or retaining ring. Both can be removed from the keg for cleaning. However, the spear should never be removed while the keg is under pressure because this can turn the spear into a dangerous projectile capable of causing serious injury.