What properties makes an object stable?

An object will be stable if the centre of mass, the point at which all of the mass of the object is concentrated, is lower to the ground. It will also be stable if there is a wide base. Objects with low centre of masses and wide bases tend to not topple. Examples of this include a car or a book lay on its cover.

Tall objects with higher centre of masses and narrow bases are more likely to topple because the line of action of weight will act outside the base and the resulting moment will cause it to rotate and topple. An upright pencil is an example of this.