Secret writing

There are many ways of writing secretly. A good spy should know at least three but we'll give you FOUR!

You will need:

Candle or white wax crayon
Paper
Water-based paint
Cotton buds
Lemon juice
Bicarbonate of soda
Red cabbage water (see instructions)
Pencil
Paintbrush

What to do:

Method 1: Candle or white wax crayon
Write your message on a piece of paper using the candle or crayon.
Now paint over the message with water based paint. See the message now? Aha!

Method 2: Lemon juice
Write your message on a piece of paper using a cotton bud dipped in lemon juice. Now either iron over the message (ask an adult to do this) or place it near a light bulb (ask an adult to do this). We don't want anyone getting burnt! See the message now? Oooh yes! And it's gone brown!

Method 3: Pencil and wet paper
Wet some paper, just a bit, and place another sheet of paper on top. Use the pencil to write your message quite gently. Take the topmost paper off. Wait for the wet paper to dry.
Now wet the paper again. Well would you believe it? There's the message.

Method 4: Bicarbonate of soda and red cabbage water
Dissolve two teaspoons of bicarbonate of soda in four tablespoons of warm water.
Write your message on the paper using a cotton bud dipped in the solution.
Let it dry.
Pour hot water over some shredded red cabbage leaves. Leave for 15 mins and then strain. You should have a dark purple solution.
Paint the red cabbage water over your message. Well I never!
Try it again but this time use lemon juice instead of bicarbonate of soda.

What's going on?

Method 1: Candle or wax crayon
Wax is oil based and oil and water do not mix. The paint will not stick to the wax message so the paint soaks into the paper and leaves the waxy areas paint-free. Your message will show up white against a coloured background.

Method 2: Lemon juice
The lemon juice is very nearly clear so does not show up on the paper when it is dry. When you heat the paper, the lemon juice starts to burn. Like all organic material (i.e. anything that was once living), the lemon juice contains carbon. When it burns, some of the carbon is released in the same way a candle releases soot. The brown writing is just the carbon that has come out of the charred lemon juice.

Method 3: Pencil and wet paper
The pressure from your pencil will mash up the fibres on the lower, damp sheet of paper. Mashed up fibres reflect the light differently to normal, unmashed fibres.
But when the paper dries, the fibres look the same, so your message disappears - until you wet the paper again.

Method 4: Bicarbonate of soda and red cabbage water
Red cabbage water is a dark purple colour. It is a natural indicator which means it changes colour in the presence of acids or alkalis. With acids it turns red or pink and with alkalis it turns blue or green. Bicarbonate of soda is alkaline and when the red cabbage water is painted on it turns blue and shows up against the purple background. If you use lemon juice instead the message will appear red against a purple background.