Centre of gravity

The centre of gravity of an object is the point at which the object can be balanced. There is an equal amount of mass on either side of the centre of gravity.

Why are certain structures easier to topple?
For an object to stay upright the centre of gravity must be over the area of support. This is the simple rule for toppling. When a person leans over to grab an object just out of reach their centre of gravity moves forward. If it moves beyond their base of support (the edge of their feet) they will topple over as shown in the animation below.