Primitive hominids are taught to make weapons by extraterrestrials.
This story has distinct echoes of 2001: A Space Odyssey which, although it is by some other fellow with the unbelievable name of Arthur C. Clarke, is much better. It also leans towards the cautionary with the ham-handed "how terrible it is that we go around killing one another"—not that I disagree with the point, but I find it rather obvious for its anemic fictional setting.