The spy who was sold a lemon

IT'S A classic line from a spy movie: Burn After Reading. For German agent Karl Muller, that advice would have really helped.

- KELLY KASULIS

Muller, a spy who masqueraded as a Russian shipbroker and covertly travelled to England among a crowd of refugees in 1915, used pen nibs dipped in lemon to write invisible messages to his counterparts. He relayed his secrets in between lines of regular ink in what appeared to be otherwise unremarkable letters.

But Muller's cover was blown when an agent from Britain's Secret Service Bureau known today as MI5 ran a flat iron over a letter he sent in 1915, causing the invisible lemon ink to show. He was executed by a firing squad at the Tower of London.