Make up a short secret message and write it on the paper with your candle. It will be hard to see but try your best. Paint over the entire paper with the watercolors. All waxes contain fats or oils. Waxes are made from petroleum oil, animal fats, or plant oils.
Watercolor paint has water in it.
As you know, oil and water don't mix. Water molecules and oil molecules are simply not attracted to each other, and so the paint will just slide off the wax. The paint sticks to the paper because it is attracted to the paper fibers. In the jargon of chemistry, the surface of sand is said to be hydrophilic , or water-loving. But there is another type of sand that behaves very differently.