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Before a vaccine is licensed in the USA, the Food and Drug Administration (FDA) reviews it for safety and effectiveness. The CDC then reviews all studies again, as well as the American Academy of Pediatrics and the American Academy of Family Physicians. Every lot of vaccine is tested before administration to the public, and the FDA regularly inspects vaccine manufacturers' facilities.
When blood is exposed to air, it clots. Heparin allows the blood to come in direct contact with air without clotting.
There are 20 feet of blood vessels in each square inch of human skin.
There are more sensory neurons in the tongue than in any other part of the body.
The first-known contraceptive was crocodile dung, used in Egypt in 2000 BC. Condoms were also reportedly used, made of animal bladders or intestines.