My butter holder arrived. I loaded it up with butter and a little bit of water. At this time of the year, my kitchen isn't much warmer (currently 6C) than inside my fridge (2C-5C range) so it won't have much of a test until the warm weather comes back. I think it works by excluding air, to prevent micro organisms, a bit like a vacuum pack. I doubt if the water offers a jot of cooling by evaporation.