This past September, Stuart Nelson Jr., the Iditarod's chief veterinarian and an experienced outdoorsman, capsized his kayak on a solo paddling trip in the Yukon. From the water, he watched as his boat—and food, shelter, and satellite phone—rushed irretrievably downstream. For 13 days, he survived on the contents of his pockets, which, because Nelson knows what he's doing, were a veritable hardware store: a folding knife, a wire saw, a signal mirror, two lighters, some Sterno (in a tiny Nalgene bottle), a space blanket, tincture of iodine, 30 feet of parachute cord, three silver spinner fishing lures, and some fishing line. This likely won't happen to you, but the lesson is clear: should you get separated from your pack or boat—or, worse, your guide and the rest of your tripmates—what you've got on your person is what you've got. At a minimum, according to survival expert Tony Nester, founder of Ancient Pathways (apathways.com), you should be prepared with what he calls "the big four": a spark rod or waterproof matches (not a lighter, which is useless when wet) to make a fire, iodine tablets to purify water, a glass mirror or whistle to signal help (this is especially important if you can't move), and, to build a temporary shelter, a pocket-size heat sheet or space blanket.