tire

I didn't create this device, so all credit to these farmers for working out a way to keep their goats water buckets from freezing in the winter, using tires, foam insulation and a bit of styrofoam.

http://www.motherearthnews.com/homesteading-and-livestock/self-reliance/how-to-keep-livestock-water-from-freezing-zm0z15djzkin.aspx