Remember the burning bat signal in The Dark Knight Rises? Wasn't it cool? Now you can do a miniature version of it, only on a piece of paper instead of a bridge. Paper is far more portable, anyway.
You can scratch a message into a piece of paper with this invisible ink, and then take a match to the paper and let the ink burn, etching your message into the paper. All it takes is a little potassium nitrate.
Potassium nitrate is an oxidizer, and supplies a large amount of oxygen to a flame, allowing any fuel to burn very quickly. You'll find it in smoke bombs, and because you find it in smoke bombs, it's tough to get outside of chemical supply stores, so here's a quick recipe that lets you make it yourself. You can find ammonium nitrate in many types of instant cold packs, while potassium chloride is available in "lite" salts. Grab 40 grams of ammonium nitrate and dissolve it in 100 milliliters of water. Filter out any undissolved ammonium nitrate with a coffee filter — this will also take out any other chemicals that were mixed in the cold pack. Next heat the solution up — but don't boil it — and dissolve 37 grams of potassium chloride in the solution. Send the solution on another trip through a coffee filter to strain it, and let it cool in an ice bath. Potassium nitrate crystals should appear on the bottom, while the ammonium chloride is swimming around in the solution. Pour off the ammonium chloride and let the potassium nitrate dry. You've got your crystals (or, again, just order them from a chemical supply store).