''Why didn't the metal solidify?'' ''Was the water toxic?'' ''Did the monster die of blisters?'' These comments from moviegoers suggest you need a physics degree to understand the ending of Alien3, the latest in the series of battles between Sigourney Weaver and a mass of death-defying organic matter; instead, it might be the filmmakers who need a refresher course. At issue? The near conclusion, in which molten metal is poured over the alien, which then surfaces Jaws-like, only to get doused by a massive sprinkler and explode into a million pieces. This confuses Northwestern University physics lecturer David Taylor, who says, ''The exploding alien defies the laws of science. If you dip a living creature into hot metal, it will burn to a crisp, even if it does have acid for blood.'' Pesky science holds that even if the creature were doused with cold water after springing out of the hot metal bath, ''it would just kind of hiss,'' says the prof. ''The monster would not explode.'' Publicists at Twentieth Century Fox, no doubt brushing up on the second law of thermodynamics (the one about reaction), were not available for comment.