There’s a good chance you already have Stanley’s Adventure Flask, which has become a bit of an industry standard for the outdoor set. It’s hardy and fine, but I recommend upgrading to the 8-ounce Master Flask, which also has a stainless-steel interior with a rustproof exterior but is curved to fit better in your pocket.
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The Stanley brand has a rich 100+ year history. Born from inventor William Stanley Jr. who forever changed the way hot drinks were consumed, in 1913 he fused vacuum insulation and the strength of steel in one portable bottle, inventing the all-steel vacuum bottle we know and love today.