A can has one weak point: its seam, the folded rim around the lid. Wear it down, and the lid lifts off by itself, with no knife and no metal shards in your food.
ACT
Look at the can first: swollen, leaking or deeply rusted, it doesn't get opened, it gets thrown away. See ADAPT.
The concrete method, the safest.
- Find a flat, rough surface: a concrete slab, a pavement, a stone, a cinder block, a brick.
- Turn the can upside down, lid against the stone.
- Rub in circles or figures of eight, pressing hard, for thirty seconds to a minute. You're wearing down the rim of the seam, not the lid.
- Check often: as soon as a little liquid shows on the stone, stop. The seam is through.
- Squeeze the sides of the can: the lid pops loose. Lift it with the tip of a knife or a spoon, never with your fingers: the edge cuts.
- Wipe the top of the can before serving: there's stone dust on it.
With a metal spoon. Hold it by the handle, the tip of the bowl against the lid, just inside the rim. Rub fast, back and forth, always in the same spot: the metal thins, then gives. Widen the hole along the rim, then lift.
With a knife, as a last resort. It's the method that injures the most. A short, sturdy blade, never a thin knife. Set the tip just inside the rim and drive it in by tapping the handle with a stone or something hard, never with your palm: your hand slides onto the blade. Then work your way around the lid with small levering moves, your other hand out of the way.
ADAPT
The can is swollen, or spurts when opened. Don't taste it, not even to check. A bulging can may hold botulinum toxin, deadly in tiny amounts, with no taste or smell. Throw it away closed, out of reach of animals. See Les conserves en bocal.
It's dented. A small dent on the body is fine. A dent on the top or bottom seam, or a sharp crease, is not: the seal may have given way.
It's rusty. Surface rust that wipes off is fine. Rust that pits the metal or has eaten through is not.
It's old. An intact can, kept dry and away from freezing and heat, stays safe well past its date: mostly the taste and vitamins fade. Acidic foods (tomatoes, fruit) go downhill faster than vegetables and meat. The state of the can decides, not the date.
The pull tab broke off. Start again from the hole it made and carry on with a spoon, along the rim.
You want to heat it in the can. Open it first, or at least pierce the lid: a closed can on a fire can burst and throw out its boiling contents. Peel off the paper label and heat it at the edge of the embers, not in the flames.
The empty can still has a use. An edge crushed flat against the stone no longer cuts: it becomes a cup, a pot or a lantern. See Making and keeping a fire for the fire.
UNDERSTAND
Why concrete works. A can lid isn't welded on: it's seamed. The edge of the lid and the edge of the body are folded into each other, then crushed flat, with a sealant inside. That rim stands a few millimetres proud of the lid. Rubbing the upturned can on an abrasive surface wears down that rim and nothing else, until the fold opens. Then the lid is held by nothing.
Why it's safer than a knife. A blade skids on smooth, curved metal, and the hand holding the can sits right in its path. Rubbing pushes nothing toward your hand, and leaves no shards or jagged edges.
A story told backwards. The iron can was patented in 1810, in England. The first can opener, in 1858, in the United States. For almost fifty years, soldiers and sailors opened their rations with a hammer and chisel, a bayonet or a rock; some cans even came with instructions to do exactly that. The metal had to get thinner before a pocket tool made sense.
What a swollen can is telling you. A can is sterilised and sealed without air. If bacteria survived, or got in through a faulty seam, they multiply and make gas: that's what pushes the lid out. The danger isn't the gas, it's whatever made it. Botulism is rare because industrial sterilisation is very reliable; when it does happen, it's serious.