We are always painting the windows in our Elementary School. I use Chromacryl....a yogurt cup full of paint and 3-4 squirts of liquid hand soap mixed in, is all that we need! It stays on the windows great and then scrapes right off when we are ready to change designs. I have the kids draw right on the windows with a black watercolor marker first and then paint over the top of their lines. Right now, each window in the Art Room has one Element of Art painted on it. When I am teaching I constantly refer to my "Window Words". I also have the Principles of Art painted on my wall....I call these my "Wall Words!"!