This part of the vacuum bottle is normally made from silicone, which fully covers the mouth of the vacuum flask to lock the heat inside. To avoid the heat escape from the bottle when opening the gasket, several screw-on stoppers are created with a press button. Once pressed, it will open a small space enough to pour out the water, the user will not need to twist open the stopper.http://www.chinaflasks.com/stainless-steel-vacuum-flask/