This icing is intended for decorating purposes only because it contains uncooked egg whites. If you'd like to use edible icing, purchase meringue powder at cake-decorating or crafts stores, and prepare icing according to package directions.
Beat egg whites at high speed with an electric mixer until frothy. Add cream of tartar and beat until soft peaks form. Gradually add powdered sugar, beating until blended. Icing should be consistency of pasty glue; it should stick to a spoon, and not pour. Add food coloring, if desired. Add water, if needed, to reach desired consistency. (If you add food coloring, you won't need to add water.)
Spoon icing into heavy-duty zip-top plastic bags; seal bags. Secure with rubber bands near top to prevent leaks. Just before ready to use, cut a tiny hole (1/8" to 1/4") in corner of each bag for piping. (If you'd rather use a decorating bag and metal tips, use a small round tip.)
Note: 1 recipe of Nonedible Icing yields enough to make 2 cookie houses.