Why do the dry beds of ponds and lakes crack?

            The bottom of lakes and ponds, which usually hold still water, is usually covered with sediments formed of clay material. Clay contains chemical substances such as sodium bentonite which have the property of expanding when wet. The loss of moisture, therefore, makes this layer of clay shrink resulting in cracks on its surface. Thus, in summers when lakes and ponds go dry, the surface of their beds shows large cracks.