Corrected entry: By the end of the movie, the squid-like alien that was surgically removed from Elizabeth grows to a creature at least 10 times the size of the original. This happens inside of the lab where there is no organic matter present. Definitely against the law of conservation of mass.
Teru_Kage
17th Feb 2013
Correction: This is consistent with the biology of the xenomorphs. In the first Alien movie, the xenomorph grew from chestburster to full size without eating any of the crew. It's possible that their physiology allows them to grow by absorbing air/water alone.
Teru_Kage