This is a great question! To answer it, we first need to think about phases of matter: Gas, liquid, and solid. Matter is made up of particles called atoms and molecules. If the particles are moving around very fast and they are spread far apart the matter is in the gas phase. If the particles are closer together and moving slowly around each other the matter is in a liquid phase. Finally, if the particles are locked in position and not hardly moving at all the matter is in a solid phase.

Water is the name of the liquid phase of H₂O molecules, and ice is the name of the solid phase.

The second thing we need to think about is density, which tells us how tightly matter particles are packed together. Tightly packed matter is more dense than loosely packed matter. Also, less dense matter floats on top of more dense matter. So when you place ice cubes in your soft drink (which is mostly water) and your ice cubes float, you know that ice is less dense than water. This is a very special property of water. Almost every other type of matter gets less dense when it changes phase from a solid to a liquid, but water gets more dense when it changes from solid (ice) to liquid. Why? Because ice is a crystal, which means it has a regular pattern with spaces in between molecules. The spaces in the crystal are larger than the spaces between molecules in the liquid. More space = less dense, so ice is less dense than water.

So now we understand why ice floats on top of water, but how does this work on a lake? Imagine that it is the beginning of winter, and the temperature has just gotten below freezing. Air changes temperature faster than water -- this is why a swimming pool seems much warmer in the evening. The air cools down at night, but the pool water remains almost as hot as it was during the day. So although the air is freezing cold, the water isn't freezing yet. The water at the very top of the lake is in direct contact with the cold air, so it freezes first. And because ice is less dense than water, the sheet of ice doesn't sink. If the temperature stays very cold, the sheet of ice gets thicker, but the ice also acts as a barrier, or insulator, between the cold air and the warmer water underneath. The sheet of ice helps the rest of the water from becoming ice!

The amazing fact that water freezes on the surface of a lake but not below it allows plants and fish to live through the winter in northern lakes and ponds. Imagine a lake without fish! So it is a good thing that H₂0 has the very strange property that its solid phase is less dense than its liquid phase.

Water is an unusual substance.When water freezes, it gets less dense. That is why ice cubes float in a glass of water. It also takes a lot of energy removal to freeze water, and the air is usually much colder than the earth at the bottom of the lake. So the cold comes from the air above the lake, and starts freezing the water from the surface. As the ice is formed, it floats and stays at the surface of the lake until a solid sheet of ice is formed. This ice prevents the cold air from reaching the water beneath it - the ice acts like a blanket for the water in the lake and helps keep it warmerthan it otherwise would be. So a lot of things cause the lake to freeze only on top. Of course, if it stayed cold enough for long enough, the lake would freeze solid.

This is one of the interesting properties of water. Ice is lighter than water, even though it is a solid form of water, so ice floats. If the lake were made of alcohol, rather than water, it would freeze first at the bottom, since solid alcohol is heavier than solid ice.

There are two reasons why.
(1) The air is the source of the cold, and the earth below retains warmth better. Thus, the surface is frozen, while underneath it is heated (or kept from freezing) from below.

(2)Ice is less dense than liquid water, and when a solid crust of ice is on a lake, the water underneath is under pressure. The freezing point drops with increasing pressure for a substance, the liquid of which is denser than the solid.