From the inside to the outside, there is a vacuum layer between the stainless steel inner tank and the outer stainless steel layer. Due to the vacuum state, heat convection can be avoided, thereby achieving the effect of heat preservation.

If your thermos cup suddenly does not keep warm, it is likely that your cup has been knocked and dropped, causing the vacuum layer to be broken and unable to keep warm.

It may also be that the sealing rubber ring between the lid and the cup is missing, and the thread of the lid is damaged and cannot be tightened.