Most invisible ink messages can be revealed by heating the paper on which they are written. The ink weakens the fibers in the paper so the message discolors (burns) before the rest of the paper.
The real secret, aside from the message, is how to reveal it without setting your paper on fire.
Tip: Don't use a lighter, match, or open fire to reveal your invisible ink message. You can lay the paper over an incandescent light bulb with fairly good results, but it is hard to tell if your paper is hot enough, so you may not know whether your paper is blank or whether you just can't see the message.
You can iron your paper, but don't use steam. This is probably the best method. A hot iron for your hair also works.
Another simple method is to wave the paper over a hot stove. If you have a secret invisible ink message, you'll start to see some distortion of the paper as it gets hot.
If you continue heating the paper, the message will darken to a gold or brown color.
If you use a stove, it's still possible to ignite your message, but it is a lot less likely than if you used fire.
Writing Instrument, Ink
Try using a broken toothpick as a pen and saliva or lemon juice as ink. You can even use plain water to write the message.
The message won't darken, but when you first heat the paper the fibers that were moved when the paper absorbed the water will bulge out a bit.