The egg riddle

The code is written on the egg shell but only if you crack and peel it you will see the code written on the egg.
Does it require an electronic device? No

We write the code with a special ink from garlic and vinegar on the shell of a well-boiled egg. The solution penetrates the egg shell and leaves on the surface of the egg a copy of the text, which is erased from the shell. The code can be read when the shell is removed.

Find it here