Fill it with your favorite mulled hot cider or cold brew, and even if you don’t get around to drinking it for hours, you can expect to find your beverage ready and waiting for you at the right temperature. The BPA-free flip cap seals tightly, so it won’t leak all over your bag, and you don’t have to take the lid off in order to enjoy your drink.
Here’s how it works: the space between the inner and outer flask wall is evacuated, creating a vacuum (well, a partial vacuum anyway…). In order for heat transfer to take place from the outside world into your cold drink, or from your hot drink to the outside world, it needs nearby molecules to excite. If there aren’t many molecules nearby, as is the case in a vacuum, the heat’s energy can’t easily be passed along, so your drink stays close to the temperature it was when you put it into the flask for a really long time.
Simply stated, vacuums make great insulation!