This is fun but needs some earlier preparation.
Take a white sheet (preferably a shiny one) and draw moon, stars and clouds using white crayon. You can be imaginative and draw anything.
Then make a watery color mixture ( as watery as possible) in a bowl. Dark colors work best. We chose black.
Then Lil P painted the paper with a big brush.. and squealed as soon as each star was revealed. The trick here is that crayons are oil based and so they resist water. Some more ideas can be found here . Have fun.