Now you can top your homebrew with a cap that will TURN BLUE when the beer is cold enough to drink! Crowns start to change to blue at 46.4 degrees and at 41 degrees they are at their maximum blue tonality indication. The return to white as they warm. Fits standard, pry-off beer bottles. Do not use twist-off bottles! These caps have an oxygen-scavenging liner that can help reduce oxidation and staling.