Among the first things a student of Chemistry learns is that nothing ever dies. It only changes its form.
There is no need for a human being to be an exception to this rule. We know that matter doesn’t get destroyed if it is burnt, but only changes its form. In the same way, man is a creature that is not subjected to annihilation. His death cannot be his extinction.
This is no indirect reasoning or empty speculation. Rather, it is a reality that is proven through direct experience. For example, cytology tells us that the human body is made up of tiny cells that are continuously disintegrating. The body of a human being of average height has around 26 trillion cells! These aren’t like the bricks of a building that remain there for a very long time. Every day, cells in the human body disintegrate in large numbers. And every day, the food we eat helps to continuously replace them with fresh cells. Through this process of the disintegration of the cells of our body and their continuous replacement, every ten years or so we receive, a totally new body, as it were, consisting of totally new cells. It is as if the hand I had used ten years ago to sign a document is no longer there but has been replaced with a new hand. And yet the signature written by my ‘former’ hand still remains my signature! Despite the change of the body, the inner person that is the real me still remains as before. My inner being remains intact. That is why it is said that personality is changelessness in change.
Source: Life, Death and Beyond