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The kidneys remove waste from the blood and make urine, also called pee. Urine is mostly water so if you don't take in a lot of fluids (or if you sweat a lot), your urine has less water in it and it appears darker. If you drink lots of fluids, the extra fluid comes out in your urine, and it will be lighter. Urine is stored in the bladder until it's peed out.