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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.
The immune system needs 9.5 hours of sleep in total darkness to recharge completely.
Eating carrots will improve your eyesight. Carrots are high in vitamin A (retinol), which is essential for good vision. It can also be found in milk, cheese, egg yolks, and liver.
Human kidneys will clean about 1 million gallons of blood in an average lifetime.
Hippocrates noted that blood separates into four differently colored liquids when removed from the body and examined: a pure red liquid mixed with white liquid material with a yellow-colored froth at the top and a black substance that settles underneath; he named these the four humors (for blood, phlegm, yellow bile, and black bile).