Highlight to read. (I couldn't resist- we were discussing invisibility, after all!)
Obviously there are invisible ink pens you can buy which are revealed by blacklight, but there are some fun other methods too! Writing with an clear acidic liquid like lemon juice will produce invisible ink revealed by heat (very Lord of the Rings!). Mixing baking soda with equal parts water creates an invisible ink which you can read by painting over it with grape juice. It's a little messier, but just as useful. If you write on a paper in white crayon, the message is invisible until you paint over the message. Then the wax will prevent the letters you wrote from being painted over, revealing the message. Finally, you can create an invisible message without ink at all. Simply take a pad of paper, like a notepad, and write with firm pressure on the top piece of paper. Remove the paper and take the one under it. It should be indented with the message you wrote above. You can reveal the message by shading or rubbing the paper lightly with a pencil. This last metho is also helpful for reading what someone else wrote last in their notebook, etc. I wish you success in your pursuit of secrecy!