"Most molecules in a cell are constantly being replaced but the DNA is not. All the carbon 14 in a cell's DNA is acquired on the cell's birth date, the day its parent cell divided. Hence the extent of carbon 14 enrichment could be used to figure out the cell's age, Dr. Frisen surmised. In practice, the method has to be performed on tissues, not individual cells, because not enough carbon 14 gets into any single cell to signal its age. Dr. Frisen then worked out a scale for converting carbon 14 enrichment into calendar dates by measuring the carbon 14 incorporated into individual tree rings in Swedish pine trees."