The lid of the thermos cup is too tight to open
Like many of the horses you learned in your middle school, the principle of pulling a vacuum copper ball is similar. You can try to soak the cup into the hot water, and then remove the negative pressure when the internal temperature rises. The cup should be opened. Be patient and try more. Heat a little more than 80 degrees, for example, because the thermos cup is not easily heated to the interior.
Usually because the cup is still hot when the lid is tight, and so on after cooling down, the principle of heat up and cold contraction, resulting in the formation of negative pressure cup, or become a vacuum, the pressure of the atmosphere pressed the lid can not open.
