Science of Identity Foundation:

Studies at the Oak Ridge Atomic Research Center revealed that about 98 percent of all the atoms in a human body are replaced every year. You get a new suit of skin every month and a new liver every six weeks. The lining of your stomach lasts only five days before it's replaced. Even your bones are not the solid, stable structures you might have thought them to be: They are undergoing constant change. The bones you have today are different from the bones you had a year ago. Experts in this area of research have concluded that there is a complete, 100 percent turnover of atoms in the body at least every five years. In other words, not one single atom present in your body today was there five years ago.²

Many people believe that a person is the brain or some part of the brain. You may be one of them. If so, the following might astound you:

Recent studies on the turnover of the molecular population within a given nerve cell have indicated that... their macromolecular contingent is renewed about ten thousand times in a lifetime.³

In other words, the matter making up each brain cell is completely renewed every three days. Your brain—that mass of matter which is contained in your skull today—is not the same brain that was in your skull last week.