I set my kids up with some paint, brushes and paper. Their directions were to paint a design on the paper, making sure to cover the entire paper in paint.
While they were beginning to dry, but were still a bit tacky, I put each of their papers on a cookie sheet and set it out in the rain. The kids had fun watching through the window as their paintings transformed, to show the little drip drops and streams of rain.
I brought them in after 5-10 minutes to dry and this was our finished product!