Royal icing is the type of "icing" that hardens as it dries on a cake. It is made from a mixture of egg whites, powdered sugar, and lemon juice or cream of tartar. The egg whites create a glossy, smooth texture, while the sugar helps the icing harden as it dries. Royal icing is often used for decorating cakes, cookies, and other baked goods, as it can be piped into intricate shapes and designs. Once dry, royal icing forms a hard, glossy surface that is perfect for holding its shape and preserving delicate details.