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 ...

A Hillhouse student’s eyes widened as he applied grape juice and heat to what appeared to be a blank piece of paper — but instead held a secret message in invisible ink, written as part of a course ...

Humans have had a long history with sending secret messages. The practice dates back to ancient Greeks tattooing a servant's head to a modern-day employee stealing corporate secrets. Invisible ink is ...