Plastic cups, glasses, travel cups or thermos cups can't be opened. This is because the air and water in the cup will expand after pouring hot water. After a period of time, the water temperature dropped, but it has shrunk, and there is a pressure difference inside and outside the cup, which can't be unscrewed. The key to opening the lid is to overcome the pressure difference.

Method 1: Turn the cup upside down, pat the bottom of the cup several times, and then screw on the lid. If not, you can soak the cup in hot water, so that when the water in the stainless steel thermos cup heats up and expands in volume (that is, the pressure in the cup increases), it is easy to open it. Or, you can put the cup in the refrigerator, take it out and pour the lid with hot water to open it easily.

Method 2: Gently pry open the gap between the cup mouth and the cup cover with metal foil to make it easy to open after air enters. Method 3: Knock with a rubber hammer or take it to the place where the hot water bottle liner is sold. They have rubber hammers and opening skills. For example, those hot water bottles with stainless steel shells can handle the dislocation of threads. Handling with a rubber hammer won't cause more damage, it just keeps the thread in place.