A Thermos isn’t just for Christmas. Our top seven vacuum flasks keep their contents freezing cold in summer as well as piping in winter We’ll stop using “Thermos” as a generic term now. After all, Thermos is just one of many brands to master the art and science of the vacuum flask, that ingenious insulated container that does the job of a portable kettle, oven or fridge without the need of a plug socket. Whether it’s a chunky “food flask”, sleek “vacuum bottle” or something in between, the job of a flask is to keep food and drink hot or cold without leaking, all day long. Some of the winners on our list can insulate for more than 24 hours, which is long enough to survive the toughest hike or most gruelling commute. For this review we’ve picked out seven flasks that excel at insulation, leak-proof seals, durability (no shattering in a single drop, thank you) and, of course, value for money. Before we reveal our list, here’s a quick shopping guide. Flasks have come a long way since your great-grandmother sent her man off to work with a Thermos of oxtail soup. Today’s best vacuum flasks are mini engineering marvels,...