It was also discovered that as time passed the nitroglycerin began to deteriorate and become unstable, which made it even more difficult to use.[15] As the grenade was a short-range weapon, users were trained to hide in a trench or other place of concealment until the tank went past them, and then to stick the grenade to the rear of the tank, where its armour was thinnest.[33] Users were relatively safe from a few yards away, as long as they were not in line with the handle when it detonated.[34] The Mark II design used a plastic casing instead of glass, and a detonator instead of a cap.[35]