Combine powdered sugar and meringue powder or egg whites in a mixing bowl and beat on low speed. Add water drop by drop. The amount depends on whether you used the powder or the egg whites, and on the temperature and humidity in your kitchen. Add the water slowly and do not let the mixture get runny – you will probably not use all of it. Beat until the mixture holds a trail on the surface for five seconds when you raise the mixer from the bowl. If you like, you can tint the icing with a few drops of food coloring. Makes 6 cups.
Note: Note: we do not recommend using raw egg whites in this recipe because of the risk of Salmonella, especially if children, elderly, or pregnant women will be eating them.