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The first-known contraceptive was crocodile dung, used in Egypt in 2000 BC. Condoms were also reportedly used, made of animal bladders or intestines.
Human stomach acid is strong enough to dissolve small pieces of metal such as razor blades or staples.
Not getting enough sleep can greatly weaken the immune system. Lack of sleep makes you more likely to catch a cold, or more difficult to fight off an infection.
All adults should have their cholesterol levels checked once every 5 years. During 2009–2010, 69.4% of Americans age 20 and older reported having their cholesterol checked within the last five years.
The lipid bilayer is made of phospholipids. They are arranged in a double layer because one of their ends is attracted to water while the other is repelled by water.