Lemon juice has been used as invisible ink for centuries. Messages written in lemon juice are invisible to the naked eye. However, when brushed with a mix of iodine and water, they become quite visible.

[youtube http://www.youtube.com/watch?v=oU2PkF9QLTQ?wmode=opaque&w=560&h=315]

You can use an updated version of this technique by modifying an HP ink cartridge so that it prints in lemon juice instead of ink. Here’s how to do it.