If you ever wanted to send secret messages using your computer, you could just embed secret messages in your image files. But if you want to print out hidden messages, you can hack your inkjet printer to print invisible messages.
The guys over at Make: put together this hack where you crack open the cartridge from your inkhet printer and replace they yellow ink with lemon juice. Print anything you want in yellow, and the page looks blank. But apply some iodine or heat the page with an iron, and your secret message will be revealed. Check out the overly complex and sloppy process in the video below:
It’s a huge pain in the ass, and looks really messy, but the results are kind of cool. Of course, you could do the same with just some lemon juice and a paintbrush or Q-Tip, but what fun would that be. This is way more geeky.
You can grab the full instructions from Make: here [PDF file].