Place the white of an egg in a bowl or plate. Add a little lemon-juice or other flavoring, and a few drops of water. Stir in powdered sugar until it is of the right consistency to spread. While the cake is still warm pile the icing on the center of the cake, and with a wet knife smooth it over the top and sides of the cake. It will settle into a smooth and glossy surface. If the icing is prepared before the cake is ready, cover it with a wet cloth, as it quickly hardens. If it becomes too stiff add a few drops of water, and stir it again. Color and flavor as desired. One egg will take about a cupful of sugar, and will make enough icing to cover one cake. If a little more is needed add a little water to the egg, and it will then take more sugar. When icing is wanted for decorating a cake, beat the whites to a froth, then beat in the sugar instead of stirring it, and continue to beat until it is firm enough to hold its form. Stirring more sugar into the unwhipped whites will make it firm enough for decorating, but the whipped icing is better. Put it into a pastry-bag with small tube, or into a paper funnel, and press it through into any shapes desired. A good icing is made of milk and sugar alone.