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Astigmatism is the most common vision problem. It may accompany nearsightedness or farsightedness. It is usually caused by an irregularly shaped cornea, but sometimes it is the result of an irregularly shaped lens. Either type can be corrected by eyeglasses, contact lenses, or refractive surgery.
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The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
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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.
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Bacteria have flourished on the earth for over three billion years. They were the first life forms on the planet.
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Normal urine is sterile. It contains fluids, salts, and waste products. It is free of bacteria, viruses, and fungi.