1. See if the tail of the bottom of the bottle is complete. If the tail is broken or knocked out, it is a disabled product, because the bottle is two layers of glass, there is no air in the middle, the tail damage indicates that the air has entered, no heat preservation effect.
2. Gently tap on the glass of the bottle with your finger. If the sound is crisp, it is a good bottle without cracks.
3. Check if the silver plating is uniform, because the silver plating avoids the heat radiation and prevents the heat from being lost.
4. Check if the bottle mouth of the warm water is round and whether the stopper is tight or leaking.
It is unscientific to choose a thermos by listening to sounds. A bottle of glass with a broken outer layer of glass, the inner layer of glass bristles will also make a "beep" sound, which is the sound of the air inside the bladder during convection.