Hot water bottles are made of double glazing, both of which are coated with silver and act as mirrors to reflect back heat rays, thus cutting off the path of heat radiation. By vacuuming the two layers of glass in the thermos, the conditions for convection are destroyed. Hot water bottle cap is not easy to choose the heat transfer cork, blocking the convection heat transfer path. By perfectly blocking all three paths of heat transfer, heat can be retained for a long time. But the thermal insulation of the thermos bottle is not so ideal, there is still a part of the heat can run out, so the thermos bottle insulation time has a certain limit.