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 ...

After captivating generations with its ingenious invisible ink puzzles, the iconic brand returns ... The hidden answers, ...

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.