Ever wondered if your bottled homebrew is cold enough to drink yet? Well take out the guesswork with these cold activated crown caps; they'll turn blue when your beer is just at the right temp! The cold activated caps crimp over standard lipped beer bottles to create a seal, keeping carbonation in and oxygen out. They also have an oxygen absorbing lining that helps to keep beer safe from the oxidative effects of oxygen in the headspace that can lead to stale beer. They are an ideal way to keep your beer fresh for as long as possible!