1, maybe the seal is not good. See if there's a gap in the bottle cap or anywhere else. If the lid of the cup is not tightly closed, the water in your thermos will soon become tepid.

2. The cup is leaking. Maybe there is something wrong with the material of the cup itself, and the technology of the thermos cup is also flawed. There may be a pinhole-sized hole in the inner container, which accelerates the heat transfer between the two layers of cup walls, so the heat is quickly dissipated.

3. The interlayer of the thermos cup is filled with sand. This is a means for some merchants to make poor quality thermos cups. This kind of thermos cup is still very warm when it is bought back, but after a long time, the sand may reflect light with the inner container, causing the thermos cup to rust and the insulation effect is very poor.

4. There is no buzzing cup in the interlayer, not a thermos cup. Put the thermos cup to your ear, there is no buzzing sound in the thermos cup, which means that this cup is not a thermos cup at all, and such a cup will definitely not keep warm.