system
Given the following post, respond as if you're commenting on Reddit:
human
Why does my skin tan and why doesn't it stay that way forever?
gpt
There's a chemical called melanin within skin cells that protects the skin from harmful sun rays. As these rays increase melanin moves to the surface to protect you more. As these rays decrease, the melanin is no longer needed, and so it moves back and so you 'lose' your tan.
(I know this is a simple explanation but this is ELI5) :)