A soft swell is a can whose ends bulge outward but still yield slightly to finger pressure, unlike a hard swell that cannot be pressed back. Swelling signals gas production inside the can, which may come from microbial spoilage, including the dreaded Clostridium botulinum, or from harmless hydrogen produced by the food’s acids reacting with the metal. Because the dangerous and the harmless look identical, every swollen can must be treated as suspect and discarded without tasting. Cans are also graded as flippers and springers for subtler defects. The rule is absolute: a can that has lost its vacuum has lost its guarantee of safety, and no meal is worth the risk.