No doubt the most satisfying way to stay warm in the winter is from the inside, with the hot beverage of your choice. But unless you're willing to boil a billy every time you want a cup of tea, you better bring along a thermos. Make that Thermos, capital T--the folks who own the company are touchy on the subject of trademarks.

I'm a second-generation Thermos man, having received my first bottle many years ago as a gift from my father. It was like a rite of passage. I was a greenhorn construction worker at the time, the youngest guy on the crew, and we were framing houses that winter, crawling along icy rafters and floor joists in the snow and wind. Dad took pity on me and gave me his best old Thermos: a battered, bullet-shaped, steel affair that was ugly as an old boot but sturdy enough to prop up a Sherman tank. It dated back to about the Korean War, which is not particularly old by Thermos standards. The company itself dates back to 1907.

The first day I took my new Thermos on the job, while I hunkered happily over a cup of hot coffee during morning break, one of the resident comedians kept me distracted with the old riddle about the intelligence of Thermos bottles--"They can keep hot drinks hot and cold drinks cold; but how do they know?"--while his buddy nailed my nail apron to the sub-floor. We were always doing things like that. Life on the ol' construction crew was a laugh a minute. Later that same day somebody bumped my Thermos and it rolled off the floor and fell into the driveway, where a lumber truck backed over it. No problem. Didn't hurt it a bit.

If you're on the kind of fishing, hunting, or canoeing expedition where it's important to minimize the weight and bulk of your equipment, a Thermos is one of those items that can be easily left behind. But for casual trips in cold weather it's always a welcome companion. Hot coffee goes great with cold mornings, and sipping a cupful while drifting down a slow river satisfies the basic need for comfort food. There's also a kind of poetry involved when the steam rising from your cup melds with the mist hanging over a winter-shrouded river. It makes a nice picture.

Thermos is a brand name, but long use and high exposure have transformed it, like Kleenex and Band-Aids, into a generic noun. There are other companies that make good vacuum-sealed bottles (I've had decent luck with a Uno-Vac, for instance, and I'm told that Nissan, of all companies, makes a good one), and the only reason I feel loyal toward the Thermos company is that theirs was the first one I owned and it kept me faithful company through some hard years. That's not to say all their products are perfect. The steel models are sturdy as you could ever want, but don't waste money on the glass-lined plastic ones: Tip them over or drop them and the liners break with a sickening shudder.

Even the best Thermos bottles can't hold heat forever. It helps to fill them first with boiling water to preheat the inside, empty them after a few minutes, then refill them quickly with your hot drink. That way you'll have hot coffee all morning and tepid coffee until late in the afternoon. And of course Thermoses works just as well for cold beverages as hot beverages, making them useful both in the summer and the winter.

So how do they know?