Why would I have an constant craving to eat ice?

I crave ice all the time. From the time I get up in the morning, I have a cup of ice handy to munch on all day long. I am ruining my teeth!! I have favorite places in our town where I know the ice in the drinks is good to crunch on. It's really strange and I just wondered why a person would have such a craving to chew on ice.