What you need 
- Light coloured paper (construction or office)
- Same colour crayon as paper
- Watercolour paints
- Paintbrush or sponge brush
- Water
How you do it
Draw a design or message on the paper with same coloured crayon. It could be your favourite animal or a secret you just want to share with your best friend. Then let the person the secret message is for brush different coloured watercolour paint over the paper. The watercolour paint will reveal the hidden message!