Decency in Movie Physics

Nerdy but fun site that spots physics errors in movies (example from Star Wars Episode I below.) And if you want to be an uber-nerd, read this excellent book critiquing the physics of Star Trek.

Not only are the physics in this movie not from around here, they're not even logical. For instance, we have a force field around an underwater city which keeps water out but which a human who is over 80% water can walk through. This same highly advanced force field technology is later used on the battle field by an otherwise primitive race of beings who use beasts of burden for transportation and catapults and spears for weapons. The catapults throw giant blue marbles which explode on contact. However, even though the primitive beings have the technology for explosives they can't seem to come up with gunpowder.