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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.
In most cases, kidneys can recover from almost complete loss of function, such as in acute kidney (renal) failure.
Cancer has been around as long as humankind, but only in the second half of the twentieth century did the number of cancer cases explode.
After a vasectomy, it takes about 12 ejaculations to clear out sperm that were already beyond the blocked area.
The average human gut is home to perhaps 500 to 1,000 different species of bacteria.