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TIL that as early as 1802, Thomas Jefferson designed a cypher wheel to encrypt messages. It's unknown if he ever actually built or used one, but a similar tool was invented for the U.S. Army to use from 1920 until the start of WWII. Jefferson's designs were discovered shortly after.

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J16bm TIL that in the United Kingdom, there is a section of a river called the Strid. It seems unassuming at hardly six feet wide, but at certain points the Strid reaches over 200 feet in depth. Air bubbles lowering the water's average density, you would sink like a rock into the violent currents below.
XEkr6 TIL that chewing gum is a synthetic rubber - polyisobutylene - and can be recycled into things like shoes, coffee mugs, pencils and rulers.
KOVYJ TIL of Oxdown, a fictitious village trainee journalists would write about in exams. In 2006 it was decided the exams would use real places instead, and so Oxdown was destroyed in a series of deadly explosions. Trainees had to report on the explosions.
mX9x TIL when a paper couldn’t think of a name for an article about Chris Hadfield they rang the Air Force who said “Just call it ‘Canadian Wins Top Test Pilot’ or something to that effect.” A friend sent Chris a copy of the article, it read "Canadian Wins Top Test Pilot or Something to that Effect”
rN5N4 TIL The word “ham” (as in HAM Radio) is not an acronym. In the early days of telegraphy & radio comms, professional operators would refer to amateur operators as “hams”, implying their incompetence. The slur was eventually forgotten and the amateur radio community widely adopted the term “HAM Radio”