The life we're living now is not our only one. We've been through many. The spark of awareness within us-- the self, or soul, or whatever you want to call it-- has no beginning and no end. No knife can cut it, no fire can burn it, no wind can blow it away.
But that spark of life keeps moving on from one body to the next. At one time you have the body of a child. Then you move on to a young person's body. Then to a middle-aged body, and then to the body of an old person. Yet in all those bodies you feel yourself to be the same you. Because you are the same you. Only your body has changed.
Your body changes, and you're always the same. So you and your body are different. You are not your body.
The final change of body is what we call "death." But it's not really final. It's only another transition, another move. Just as you change from childhood to youth to old age, at death you move on to go through the cycle again, with birth in another body.
What we do in this life sets us on our way to the next, just as what we do in school gets us ready for where we go when our schooling is over. In our next life we can go up, or we can go down. Or we can get free from the cycle altogether.