Initially, this specially designed 1-liter flask should be in a normal upright position and partially filled with water. After bringing the water to a boil, close off the neck with a rubber stopper fitted with a thermometer and invert the flask. Place ice in the special concave bottom. As you do so, the water starts boiling rapidly again. The thermometer clearly shows the temperature is falling, but the water will boil until it is only 15° or 20° above room temperature. Includes rubber stopper with hole and thermometer. Requires Iron Support Ring with Clamp, approx. 12cm I.D., not included.