What makes a flask worth your money is its ability to slow down heat transfer – either in or out – via a layer of almost airless space sandwiched between the flask’s inner and outer walls. Depending on their size and performance, the best flasks can keep drinks hot or cold for hours, sometimes an entire day, and sometimes even longer.
There are 1l giants with integrated cups, which hold at least several cups of tea to last a cold hiking trip, and mid-sized yet compact vessels for hiking. Then there are coffee-cup-sized flasks to slip in your bag for the morning commute. Some flasks are better suited to drinking cold drinks from, owing to the open design of their mouthpiece.
