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TIL That levels are sometimes referred to as "spirit levels" because the fluid inside was commonly colored spirit or alcohol for contrast. Today the fluid is usually a dyed ethanol because it doesn't freeze as easily and provides less friction for bubble movement when compared to water.

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OGkBL TIL: Ancient Chinese Toys (400 BC) Inspired Modern Helicopters. A stick with feathers at the end was rapidly spun between the hands to generate lift and then released into free flight. In 1754, Lomonosov developed a rotor modeled after the Chinese top, but powered by a wound-up spring device.
GAXd7 TIL that Sigmund Freud's first research was looking for the existence of testes in eels
X0J68 TIL in the 1980s, NASA had a 1-900 number which charged $2 for the first minute and $.45 for each additional minute. It allowed callers to listen in on a mission status report and mid-flight press conferences, and thousands of them heard the Challenger explosion in real time.
b95Ya TIL that the north star is 4000 times brighter than our sun. The light we see when we look at the north star was generated in the year 1587, and it has been traveling through space for 434 years to reach us.
GX9j TIL that you’re never actually touching anything. Your atoms’ electrons repel objects when they’re 10^-8 meters away from you, but you can feel the force of the resistance.