Water Tower

We finally have some sunshine today. The first time for about two weeks. We have a majestic water tower down the road from us. It dominates the landscape for quite some distance. I've always had a bit of a fascination for these things. They come in lots of different designs. How come they don't freeze when the temperature drops below zero? We have had freezing temperatures all week, but never any problem with the water. Well done Water Board!