Mark Smith, age 13, of Omaha, Neb., for his question:

WHY DON'T LAKES FREEZE SOLID?

A mountain glacier is a frozen river, and a great icecap is a frozen freshwater sea. However, these solid masses of ice do not form in the same way as the winter skating rinks form on your favorite lakes.

It's interesting to imagine what the world would be like if a few simple laws of chemistry were changed. For example, suppose ice sank. With the first winter frost, carpets of solid ice would begin to form on the floors of our lakes and streams. Each frost would add to the deepfreeze, and come spring at least some of our waterways would be frozen solid from top to bottom.

However, the ice always forms a crust on the surface of wintry waters. The delightful skating rink process begins when the temperature of the air dips to, or below, 32 degrees Fahrenheit. Assuming that the air temperature stays around this freezing point of water, with no warm spells to melt the ice making project, the icy crust will become thicker and thicker. Before winter ends, the ice on top of the Great Lakes becomes several feet thick.

But, strange to say, none of this ice sinks to the bottom. There are at least two main reasons why this is so. One is the different heat holding quality of air and water. The other is the fact that solid ice is less dense, and therefore lighter, than water.

As the sun's radiant energy streams down through the atmosphere, it tends to save its warmth to shed on the land and water areas of the earth's surface. The surface warms the lower level of the atmosphere. However, the land absorbs this heat faster than water and also lets it go faster. Come winter, as the land becomes chilly, the water retains some of its summer heat.

With the first frost, air chills the surface of the lakes forming an icy crust. The water at lower levels still retains some stored warmth  and the solid crust of surface ice tends to protect it from the freezing air above. Thus a slight frost is enough to form a thin surface crust. But a season of prolonged severe frost is needed to form very thick layers of ice on the surface.

A glacier, frozen solid from top to bottom, is another story. It forms when the summers are too mild to melt the accumulation of winter snows. Year by year, the heavy piles of frozen snow build up solid masses of impacted ice on the ground.