I didn't want to cloud this very good thread (https://www.backyardchickens.com/forum/viewtopic.php?id=100552) about solar pop can/passive heating up with these questions, but I saw a different topic emerge there that I've been chewing on for the last year almost. Heating the water, or at the least keeping it from freezing. I've been thinking of hot rocks, placed inside an insulated metal barrel/bucket and simply setting the waterers on that. The victorians used these metal skillet thingys and put hot coals in their beds to keep them warm at night... this seems similar to my pea brain. Maybe if you have a fireplace hot coals would work better... DH thinks the rocks will cool off too quickly... But a soup thermos keep soup HOT for a LOOOONG time... mine will keep my coffee hot for at least 5 hours! What do you think?