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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.
Blood is approximately twice as thick as water because of the cells and other components found in it.
Blood in the urine can be a sign of a kidney stone, glomerulonephritis, or other kidney problems.
Egg cells are about the size of a grain of sand. They are formed inside of a female's ovaries before she is even born.
The modern decimal position system was the invention of the Hindus (around 800 AD), involving the placing of numerals to indicate their value (units, tens, hundreds, and so on).