Sugar not only sweetens the meringue but also gives it a sturdier structure. Use superfine sugar and add to the egg whites gradually. Beating the egg whites too quickly or at too high a speed can form unstable air bubbles, which can collapse. Whip the egg whites at low speed until they become foamy, then increase to medium speed. The acidic cream of tartar helps create a more stable foam. Add cream of tartar or an acid such as lemon or vinegar to help the meringue hold shape and to keep it from weeping.