Wonderful slice of history in this Mental Floss article on 11 Historical Uses for Invisible Ink:

James Stockdale, a Navy pilot, was shot down over North Vietnam in 1965 and sent to the ‘Hanoi Hilton,’ where he was to stay for seven and a half years. With the help of U.S. Naval Intelligence, his wife Sybil initiated secret communications with Stockdale by enclosing a photograph of her mother in a letter to him. He was confused, but (as he said later) he thought: ‘It’s dumb to throw away something from the States without doing more with it. James Bond would soak it in piss and see if a message came out of it.’ So he did. After it dried, print appeared on the back, establishing the code that he later used to communicate with the Navy, informing them of conditions at the prison.

And now I’m thinking … is that what James Bond would do? Has he ever done that in print or on screen? When he’s in a jam does he just start soaking things in piss?