If you want to make sure that your cold drink stays ice cold, or your warm drink can still warm your bones after a few hours, there is a bit of preparation that you need to do. Thankfully, it’s a fairly simple process. Depending on what your desired temperature is, you’ll need to either pre-heat or pre-chill the thermos bottle. First, if you want to pre-heat the bottle, open it up and pour in some boiling water.
Let it stand for a few minutes. This allows the heat of the boiling water to radiate into the trapped air, heating it up slightly. That way, any heat lost from your drink will be replenished by the heat trapped in the double wall construction. Conversely, if you want to make sure your drink stays cold, allow very cold water to sit in the thermos for a few minutes to chill the air or add ice cubes to the thermos prior to use.