Forgetting a can opener when you pack for a picnic or camping trip does not automatically mean a last-minute change of menu. You can use common kitchen utensils such as a knife or spoon or an army-issue tool to open a can at its point of weakness; you can even break the seal with some deftly applied friction. In all cases, beware of razor-sharp edges, which need to be handled with care.
The seemingly inoffensive spoon can rip a lid from its can, but you need to wrap the handle in a cloth or towel or wear heavy gloves, as the technique takes its toll on your palms. Hold the spoon upright, steadying the tip of the handle with your thumb and gripping the shaft firmly. Rub the concave spoon end furiously against a small section of the inside lid until it breaks through. Once you have an opening, insert the spoon and nick at the can toward yourself, rather than away, until you complete a circle. For best results, use a heavy, thick spoon that won't bend under pressure.
Generations of servicemen worked their way through a variety of canned C-rations with just a sliver of metal instead of a cumbersome can opener. You can find an Army-issue P-38 or P-51 can opener from a military surplus store and master the technique in minutes. The Army can opener has a flat handle that you hold by the thumb and a hinged blade. Hold the handle notch tightly against the outside lip of the can and puncture the inside with the blade, working your way around the inner edge.
The same technique works with the Swiss Army knife blade; use heavy leverage with small, precise strokes. The secret is to hold the can firmly to prevent warping, as this would allow the lid to buckle upward and make it more difficult to cut.
The heavy blade of a sharp chef's knife is more than a match for the thin skin of the typical aluminum can. Either hold the knife with your palm on its blunt edge where the blade meets the handle and push the protruding heel into the can, nicking it inward with each downward stroke, or take the tip and strike it down vertically to make a hole. Lower the handle, turn the knife upside-down, and rock the knife upward with small strokes to nick away at the can.
You need to use one hand to hold the can still, and you must make upward slices. If the knife snags and lifts the lid upwards, you lose a lot of the structural advantage and might end up forcing the blade. In both cases, make sure the blade is sufficiently sharp and heavy as you don't want to force the blade while holding a can with sharp edges. Note too that your chef knife will definitely be worse for the wear following this method.
If you're camping and truly desperate, you can invert the can and rub the lip vigorously on a stone in short, back-and-forth strokes, applying sufficient downward pressure. The friction gradually breaks down the crimped seal on the can until you turn it the right-way up and lift out the lid or squeeze the can to pop it free. A trickle of liquid is a sign that you are making progress, but it also means this method doesn't work so well for cans of soup, broth or coconut milk, for example. Use only clean stone surfaces, and sluice the surface down afterwards to remove spillage.