Dying village teacher's pupils save Earth
A teacher in a poor mountain village is dying of cancer and spends his last days drilling his pupils in Newton's three laws of motion, making them memorize the laws even though they barely understand them. Meanwhile an alien war fleet is clearing a buffer zone by destroying stars, and it spares only systems whose inhabitants pass a test of basic civilizational knowledge. The fleet happens to sample these children, and because they can recite the laws, the Sun is spared and the teacher never learns what his lessons achieved.