As a child I vaguely remember reading that you could write letters in invisible ink made from onion juice. I believe the writing was revealed by holding the paper close to a heat source. It’s so long ago now I can’t quite recall if I actually tried it out.
This memory was pulled from the darkest recesses of my memory today when I read a story in The Sun, always a good source of inspiration if not quite truthful reporting. Lol
I just read that when MI6 were researching invisible inks it was discovered that semen was just the thing for the job. Don’t laugh I kid you not. Apparently the story behind this somewhat unusual writing medium has been documented in a book documenting the history of the secret intelligence service.
I’m just wondering how many volunteers I could find to keep me in supplies…