Who knew how useful a lemon could be?

In World War I, several so-called “lemon juice spies”—German agents operating in England—used citrus as their means of communication. The British government had stepped up its censorship of letters in wartime. One agent, Mabel Beatrice Elliot, flagged letters written by three of these men, heated them up, and unmasked them as spies. The lemon-juice operation was a clumsy one: several spies, once caught, had lemons on their persons, or pens with pulp still stuck on the nibs. In the end, the British executed 11 German spies in the Tower of London in 1915; four of them had used lemon juice. “After the painful and visible loss of … the lemon juice spies,” Macrakis writes, “the Germans began to develop more sophisticated invisible ink methods.”