How can we use the half-life of Carbon-14 to date things?
All living things are exposed to cosmic rays while they are alive. The cosmic rays keep the percent of Carbon 14 in the body of living organisms basically constant (let's say 10%) When the living thing dies they are no longer able to absorb cosmic rays so the percent of Carbon 14 in their bodies begins to decrease. The half life of Carbon 14 is about 5700 years. That means if a living thing dies with 10% of it's carbon as Carbon 14, in 5700 years, only 5% of it's carbon will be Carbon 14. Based on how much Carbon 14 in the once living organism is left, scientist can tell how long ago it died.