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There are more nerve cells in one human brain than there are stars in the Milky Way.
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.
Human stomach acid is strong enough to dissolve small pieces of metal such as razor blades or staples.
There are more bacteria in your mouth than there are people in the world.
When Gabriel Fahrenheit invented the first mercury thermometer, he called "zero degrees" the lowest temperature he was able to attain with a mixture of ice and salt. For the upper point of his scale, he used 96°, which he measured as normal human body temperature (we know it to be 98.6° today because of more accurate thermometers).