Yes, it really can help the thermos maintain temperature. And prechilling works for cold liquids as well! For one thing, when transferring hot liquid from one container, like a coffee pot, to the thermos, preheating helps reduce the amount of heat lost from the liquid during the transfer. To preheat your thermos, pour boiling water into it and let it sit for 30 seconds to a minute. Pour out the boiling water and replace it immediately with the hot liquid of your choice.;