It's the Most Famous Lemon in Spy History

It's the stuff of spy novels, except in this case it's the stuff of actual spycraft. A new exhibit opening in London features a 110-year-lemon—now desiccated, of course—used by a German spy to write messages in invisible ink about British troop movements during World War I, reports AFP. Shipbroker Karl Muller wrote his secret lemon-juice messages between lines written in ordinary ink on seemingly humdrum letters. As the Washington Post explains, he was busted in 1915 when an agent from Britain's spy bureau, now called the MI5, ran a warm flat iron over the letter and made the invisible words visible.

Muller was executed in the Tower of London in short order. The lemon is one of the items in a new display called "MI5: Official Secrets" at the National Archives in Britain. When arrested, Muller had a lemon in his overcoat pocket, and he tried unsuccessfully to convince police that he used it to clean his teeth. "Lemon juice had been used for centuries as a form of 'invisible ink', since it cannot be seen on paper until it (carbonizes) when heated," explains the National Archives in a post about Muller's case. More sophisticated methods were available by 1915, but Muller was apparently unaware of them. (More strange stuff stories.)