We are expecting a hard freeze tonight. The water in our bird baths will probably have a layer of ice, if they don't freeze solid. If cold water sinks, why does the ice form at the top?
Water is a very strange chemical. As water gets colder, it gets denser, and sinks to the bottom. The strangeness comes in when the water cools down to 39°F. At that point it starts getting less dense, and floats back up to the surface where it freezes to form the layer of ice. This property of water is due to the way water molecules interact with each other, and is only seen in a few other chemicals.