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There are over 65,000 known species of protozoa. About 10,000 species are parasitic.
There are more nerve cells in one human brain than there are stars in the Milky Way.
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.
Nearly 31 million adults in America have a total cholesterol level that is more than 240 mg per dL.
As the western states of America were settled, pioneers often had to drink rancid water from ponds and other sources. This often resulted in chronic diarrhea, causing many cases of dehydration and death that could have been avoided if clean water had been available.