This is Art and Science really and great to try at any age...
It's super simple all you need is...
...Some white paper, a white crayon, some paint, a paint brush and perhaps a little water.
First draw on your white piece of paper with your white crayon. Harmonie, my 3 year old, said 'Oh, no! Not working.' Then I gave her some paint and a paint brush and let her paint over what I had helped her draw. She found the picture and oh the excitement, it was so sweet.
Why does this happen?
The science behind this is why it works. The answer is Wax. Wax is a Waterproof substance, it is hydrophobic. Water has a high level of surface tension, it's molecules cohere (cling to each other to form a whole) strongly and only adhere (stick to) other things weakly.
This magic writing is a great example of this as when we paint over the wax, we can see the water beading up on top of the waxy surface and then rolling away on to the unwaxed paper. This is what leaves us able to see the what-was-once-invisible picture.
Other places we see this same thing happen is on the top of waxy leaves or on waxed cars.
It is because the surface tension makes these water drops round so they cover the smallest possible surface area. This is, as I've already said, because wax is hydrophobic (a water hater).
Older children might want to use this to write secret messages and then this project can lead on to trying out some invisible inks - always a great hit with my girls.
This is also a great project for children who don't like to write much as it makes writing so much more fun - you can write anything you like of course, if not just draw a picture!