Corrected entry: They try to kill the alien by covering it in molten lead. Lead is one of the heaviest elements we know about. Unless the alien and the guy it was eviscerating were made of a denser material than lead (unlikely, since we see it cling to flimsy ceilings etc) both the alien and its prey would have bubbled to the top of the lead, much as a balloon would rise to the surface in a swimming pool.
Jon Sandys
15th Oct 2004
Correction: There are several things wrong with this: (1) the lead was literally burning through their flesh, and considering the temperature of molten lead (327.5 °C, 600.65 °K, 621.5 °F) would have happened extremely quickly, (2) in order for something to float, the buoyant force has to overcome the viscosity of the liquid, and even molten lead has a high viscosity, (3) the volume of lead could compress the material of the bodies to such a density that it was not buoyant.