system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is your urine yellow when you don't drink enough water?
gpt
Your kidneys are constantly filtering waste products from your blood.  They do this by dissolving the waste materials in water, and sending the waste-materials-plus-water, which is called urine, to your bladder, so you can pee it out.

When you don't drink enough water, your kidneys end up having to dissolve the waste products with less water, which makes your urine more concentrated, and thus darker yellow.