William Abel, a mail censor at Iowa's State Prison, Fort Madison, using a hot iron to bring out a secret message written in lemon juice on a letter.Three Lions/Getty Images Humans have had a long ...

For a long time, seeing those words pop up onscreen was the best indicator of good gossip to come. Close seconds included “OK, so…” for the more demure among us, or a simple teacup emoji for the ...

Which invisible ink pens are best? Spies and conspirators have used invisible inks for millennia to send messages. In the modern age, invisible ink pens are an excellent way for grown-ups to ...

Jake Peterson is Lifehacker’s Tech Editor, and has been covering tech news and how-tos for nearly a decade. His team covers all things technology, including AI, smartphones, computers, game consoles, ...

The secret history of invisible ink. American Expeditionary Forces secret ink lab By signing up, you confirm that you are over the age of 16 and agree to receive occasional promotional offers for ...

A ninja history student in Japan sent her education into stealth mode when she turned in a seemingly blank paper for her term essay. Eimi Haga wrote the essay with invisible ink. Haga told the BBC ...