I’m thinking it’s a conflation with a basic “invisible ink” made with lemon juice.

You write a message in lemon juice on paper; the juice dries clear, then you can heat the paper to darken the juice and make the message legible.

Wow. What a resource. Thanks for putting this together in such a sharable format!

Do the students call you Principal DaddysLittleSlut?

Are you thinking of r/ParlerTrick?

Liang hadn’t made his hyphenation algorithm yet :(