Your skin replaces itself roughly once a month.

The outermost layer of skin, the epidermis, is constantly renewing itself. New cells form in its deepest layer and gradually get pushed upward, flattening and hardening as they go, until they reach the surface as dead, flattened cells that eventually flake off. In younger adults, this whole cycle takes roughly a month; the pace slows with age, sometimes stretching to six weeks or more.

That turnover means the visible surface of your skin today is entirely different material from what was there a month or two ago. It’s also part of why superficial scrapes and sunburns heal on their own over a matter of weeks — the damaged surface layer is simply replaced from underneath.