It may exist for a few seconds or an hour, a day, a week, whatever.You can use any materials you like, natural or man made. It could be a drawing in the sand that the waves wash away, ice that melts, a sculpture in the snow, or painting with with water on a stone like Invisible DrawingMake sure you document your work as thoughtfully and beautifully as you can. Remember the documentation ( photo or film) is what remains...