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TIL Leo the Mathematician devised a network of communication beacons that was built and operated around 1100 years ago. Based on modern experiments, the network's several branches could transmit and receive messages at a speed of 450 miles per hour

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7rJ7Z TIL the Stasi, East Germany's secret police force, employed around 90,000 full-time staff, assisted by around 170,000 unofficial informants and countless other occasional collaborators. It is estimated that there was one agent or informant for every 6.5 people living in the GDR.
KRB4 TIL the use of sunscreen can cause coral bleaching; the main component of sunscreen, titanium dioxide, forms hydrogen peroxide, OH- and superoxides (O2-) in water, all of which contribute to coral bleaching. 10% of bleached coral is estimated to have been caused by sunscreen alone.
ZpWDa TIL of a 1950's doctor who transfused his sick patient's blood into his own body to understand her condition better. Impressive and scary!
6KPl TIL that the inventor of the Kaiten manned suicide torpedo, Lt. Hiroshi Kuroki, was killed during one of the first training missions. When his torpedo was raised, a note was found that both sent respects to his family, and detailed the cause of the accident and how to fix the defect.
x940 TIL Helsinki, Finland has a four-times-a-year event called Restaurant Day. The occasion offers anyone the opportunity to set up a restaurant, café, or bar, for just one day, without having to apply for official permits – as long as alcohol is not on the drinks list.