The line of action of the weight of an object acts through its centre of mass. If the line of action of the weight is outside of the base, there will be a moment and the object will topple over. You can make objects more stable by giving them a wider base or a lower centre of mass; this works because these things mean that the object has to tilt further before the line of action of the weight is outside of the base.
yh. pretty picture right?