Center of gravity + torque

Although the Leaning Tower of Pisa is leaning it does not fall down. This is because the tower's center of gravity is over, or within the base of support. Center of gravity is when gravity acts on a center of mass. The center of gravity is the average position of mass in an object.
In the picture below, the pink dot is the center of mass, the pink line is the center of gravity and the green is the base of support.  Although the object is leaning it is not falling over because its pink center of gravity is within the green base of support. In the second picture the pink center of gravity is not within the green base of support. Therefore the object will have a blue force resulting in an orange lever arm. A lever arm is the distance from the axis of rotation, or where the object rotates from. When something falls it rotates. In order for something to fall, torque is required because it causes rotation (torque=forcexleverarm). In order to have a large torque, the object must have either a large force or a large lever arm or both. The larger the torque, the larger the rotation. In the situation below, the object falls because its center of gravity (pink) is outside the base of support (green), has a lever arm (orange), and therefore has a torque which causes the object to fall.