The Pilot's Invisible Ink
A WWI prisoner of war became a master spy from inside his cell.
After being shot down in 1917, French pilot Lt. Édouard de Laleu was held in a German POW camp. Using invisible ink made from lemon juice, he wrote postcards home detailing crucial intelligence about German troop movements. The seemingly innocent notes were sent to a secret address at the French War Office, where the messages were revealed by heating the cards.