Ice is an amazing substance!! It defies the laws of physics. Not only is the solid phase of water amazing, water itself is also amazing. They both have traits that break the rules.
Let me start with water. You have been taught for years that heat rises. Just look at a hot air balloon. The warmer something gets the less dense it is and therefore lighter. Water acts in a similar way by becoming more dense as it cools until it reaches 39.2 degrees Fahrenheit where it starts to expand instead of shrink. Water is most dense at this temperature, so as it cools down toward the freezing mark, it become less dense and heads upward to the surface.
At first look, this doesn't make much sense, but if ice froze from the bottom up, it would be a bad situation for aquatic life. Even worse, since the surface is exposed to the cold air, it would freeze first and then sink to the bottom taking everything with it. God and mother nature have changed the physical properties of freezing water to preserve life in ponds and rivers. The solid phase of water (ice) is less dense than the liquid phase. It also makes it easier to ice skate if the ice is on the top (grin). Now I hope there is no ice on your ponds this late in the season but only in your drinks. You all have a great day today, you hear?