Cosmic rays knock out neutrons in the atmosphere which then collide with nitrogen to form radioactive Carbon-14. These are incorporated into living plants through uptake of carbon dioxide in photosynthesis, but when plants die the carbon is fixed in the plant tissues. No more carbon dioxide is taken up and any carbon-14 present decays into nitrogen. As a result the proportion of carbon-14 in plant tissues decreases.The half-life of carbon-14 is 5730 years, so the proportion of carbon-14 in dead plant tissue, and hence the activity, gives an indication of the age of the organic material.