Right now, 40,000 feet overhead, a cosmic ray is sending a neutron smashing into a nitrogen atom, smacking a proton out of its nucleus and forming an isotope called carbon-14. Armed with the equation below, archaeologists use these atoms to pinpoint how old the Dead Sea Scrolls are, or the drawings in Chauvet Cave, or Ötzi the Iceman.

Living things constantly consume carbon—through photosynthesis, for plants, and for animals, ingestion of those plants. The atmospheric ratio of carbon-14 to regular carbon-12 remains consistent at one part per trillion, so if something is alive, one-trillionth of its carbon atoms will be C-14. But once a plant or animal dies, its carbon-14 is no longer replenished. C-14 is radioactive and unstable, with a half-life of 5,730 years, which means that half the atoms will turn back into nitrogen over that period. That rate of decay is key to gauging age.