Meringue
Meringue is a whipped mass of egg whites and sugar. Whipping turns the white into a foam: the protein molecules unfold and arrange themselves around air bubbles, while the sugar stabilises the structure by binding water and stopping the foam from collapsing.
There are three classic types. French — raw whites whipped with sugar. Italian — whites cooked by hot sugar syrup. Swiss — whites and sugar warmed over a bain-marie and then whipped. Each gives a different density, stability and range of uses.
Meringue is judged by how far it is whipped. Soft peaks flop at the tip of the whisk; stiff peaks hold. Over-whipped meringue turns grainy and begins to separate: the protein network tears and releases its water.
The enemy of meringue is fat. Even a trace of yolk or a film of grease on the bowl will stop the white forming a stable foam.