system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the sun cause long term skin damage if we are constantly losing and re-growing skin cells?
gpt
Because we're only losing and regrowing the outermost layer of skin, and near UV radiation can penetrate to the innermost layer of skin where cells are alive and well and furiously producing more skin to keep up with the loss. 

If a single UV photon hit one of these cells in the wrong spot as it's preparing to split (which is how more cells are produced), this can cause it to start misbehaving, and be the start of cancer. 

If this cell is not a regular skin cell but a pigmentation cell, it's even worse since those are able to metastasize even without unintended mutation. Now you have cancer that can spread throughout your body. 

Edit: just to clarify, the UV photon has to hit *exactly* the right spot at *exactly* the right time to cause cancer. The risk of any one photon doing so is extremely small, but the number of photons in sunlight is extremely large which is why it happens sometimes.