The 'intelligent' silicone plug makes sure that air-pressure between the two layers of glass is always adjusted. Our insulated, double wall glass is handmade by expert artisans. Because this glass is mouth-blown, each one may have slight differences in height, thickness & weight. Please use care when placing ice into the glass to avoid fracturing the inner layer. We suggest placing the liquid first and then the ice. Please also avoid using metal spoons or placing the glass into a crowded dishwasher. These glasses will help to keep hot drinks warmer and cool drinks cooler. For best results retaining heat, pre-heat the glass before use.