Have you ever written a message using invisible ink?
You can easily make your own ink to do this. It's quite easy! All you need is some milk from the fridge, and you can write on a piece of paper using a fine paint brush or a cotton bud.
While the milk is still wet, you can easily read it because it is still shiny, but once it is dry no-one can tell that it's there at all.
To read the secret message, you have to heat up the page. You can do this over a candle, but be very careful you don't let the paper catch alight! Do it outside, and have some water nearby, just in case. You can also do it by ironing the page, or by putting the paper in a warm oven for a few minutes. Be careful! If the oven is too hot, the paper might catch alight and you'll lose your message!
You can also use lemon juice as invisible ink.