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Your skin wrinkles if you stay in the bathtub a long time because the outermost layer of skin (which consists of dead keratin) swells when it absorbs water. It is tightly attached to the skin below it, so it compensates for the increased area by wrinkling. This happens to the hands and feet because they have the thickest layer of dead keratin cells.
The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
A seasonal flu vaccine is the best way to reduce the chances you will get seasonal influenza and spread it to others.
In most cases, kidneys can recover from almost complete loss of function, such as in acute kidney (renal) failure.
Normal urine is sterile. It contains fluids, salts, and waste products. It is free of bacteria, viruses, and fungi.

