|Image courtesy of Jeff the Gardener|
That mortar shell was, of course, the classic Stanley flask. My father's is still in better shape than this one, but it takes quite a bit of abuse to destroy one of these things. And note the specification: it'll keep your beverage at temp for 24 hours.
The stainless flask is truly a brilliant device. It uses no energy to maintain temperatures -- it's just metal, impervious steel, odorless and tasteless. The liquid space is nestled in an evacuated cavity so the only heat transfer happens through the metal itself. The metal only connects at the neck of the bottle, which can only transfer heat slowly -- over 24 hours or so.
Glass-lined vacuum flasks (like Thermos) are perhaps even more efficient. However, they're not as durable, and if you've ever broken a glass vacuum flask, you wouldn't want to do it again. So it's stainless all the way for me.
Before we go, I'll leave you with a few other high-fliers on the stainless flask scene:
The Thermos Nissan 34 oz.
The Zojirushi Tuff Slim (note detailed temperature performance specs)
Or, if you're cheap like me...Ikea always has one or two very affordable options.