Hearts

Apparently you don't die with the same one you were born with. Here is how they figured it out:

Tests of nuclear weapons in the atmosphere, which lasted until 1963, generated a radioactive form of carbon, carbon-14. The carbon-14 in carbon dioxide is breathed in by plants, turned into glucose (see equation) and enters the human diet. In the body, the carbon-14 is incorporated into new DNA, and once a new cell is made, its DNA does not change. The level of carbon-14 in the atmosphere has dropped each year since 1963 (see graph), so the exact amount in a cell marks the year the cell was born. From a cell's birth date, researchers can calculate how quickly different tissues such as the intestine, brain and heart are renewed.