A startled pufferfish doesn’t gulp air. It swallows water, dozens of rapid mouthfuls, and transforms from a slow swimmer into a rigid, near-unswallowable sphere in seconds. The mechanics are stranger than the result. The engine is buccal pumping. The fish seals its gill slits, then uses the floor of its mouth like a piston, drawing water in and forcing it back into a vastly expandable stomach. A one-way valve at the esophagus keeps it from escaping. Each pump adds volume; the animal can swell to several times its resting size. What makes this possible is tissue architecture: - The stomach has abandoned digestion entirely, becoming a folded, balloon-like sac with pleated walls that unfurl under pressure - Pufferfish skin lacks scales and is built in two layers laced with a dense collagen network, slack and corrugated at rest, that straightens rather than tears as it stretches - They have no ribs and no fused pelvis, removing the skeletal cage that would otherwise limit expansion Inf...