I heard a very cool story on Qi, and it goes: Arthur Conan Doyle, the author of Sherlock Holmes, sent cards to his friends saying “We are discovered, flee now” as a joke. And one of his friends really disappeared!

What a clever and funny idea, thought I.
So I made a card that you can download and print and send to your friends saying the same. See which of your friends is actually the secret member of a covert organization.

Download printable pdf here.