Ice floats because it is lighter than water. When water freezes, it increases its volume (expands) by about one-tenth. When 10 quarts of water freeze, then. 11 quarts of ice form. Thus, ice is lighter than water and will float. This expansion of ice also explains why one-tenth of a floating ice cube is above the surface. Ice decreases about one-tenth in size as it melts. Therefore, the water from a melted ice cube will take up only as much space in the glass as the cube took up when it was floating. So the water level in a glass doesn't rise as the ice melts.