Why cant meringue be uderwhipped or overwhipped?

Meringue can’t be underwhipped because it won't achieve the necessary stability and volume, resulting in a runny texture that cannot hold its shape. Conversely, overwhipping can cause the proteins to become too stiff, leading to a grainy texture and risking separation of the liquid (weeping). Proper whipping creates a stable foam that is both light and airy, essential for successful meringue.