Spy used pen nibs dipped in lemon to write invisible messages to his counterparts. He relayed his secrets in between lines of ...

Burn after reading.” For German agent Karl Muller, that advice would have really helped. Muller, a spy who masqueraded as a Russian shipbroker and ...

A 110-year-old lemon used by a German WW1 spy to send secret messages from Britain with invisible ink is going on display in ...