the tracks for a "mobility kill" (m-kill), or target all external visual aids with rubbery cohesive substances such as melted rubber or blackened high viscosity epoxy resins. Once a tank is disabled it is easier to destroy. This is why side-skirts are an important feature; they can deflect heavy machine-gun bullets and trigger the detonation of high-explosive anti-tank (HEAT) rounds before they strike the running gear. Other vulnerable parts of a typical tank include the engine deck (with air intakes, radiators, etc.) and the turret ring, where the turret joins the hull. When used defensively, tanks are often sunk into trenches or placed behind earth berms for increased protection. The tanks can fire off a few shots from their defensive position, then retreat (reversing) to another prepared position further back and drive behind the berms or into the trenches there. These positions can be constructed by the tank crews, but preparations are better and quicker if carried out by combat engineers with bulldozers. Overhead protection, even if it is fairly thin, can also be very useful since it can help pre-detonate artillery shells and avoid direct hits from above which can be deadly to tanks, by striking them at their thinnest armour. In short, tank crews find as many ways as possible to augment the armour on their vehicles. Tanks usually go into battle with a round in the gun, ready to fire, to minimise reaction time when encountering an enemy. The US doctrine calls for this round to be a kinetic energy (KE) round, as the reaction time is most important when meeting enemy tanks, to get the first shot (and possibly the first kill). If troops or light vehicles are encountered, the usual response is to fire this round at them, despite it not being ideal—it is difficult and time-consuming to remove a round which is already in the breech. In this case, after the KE round is fired, a HEAT round would normally be loaded next to continue the engagement. Tanks can be decisive in city fighting, with the ability to demolish walls and fire medium and heavy machine guns in several directions simultaneously. However, tanks are especially vulnerable in urban combat. It is much easier for enemy infantry to sneak up behind a tank or fire at its sides, where it is most vulnerable. In addition, firing down from multi-story buildings allows shots at the thin