Here's the unsettling part about a jellyfish sting: the jellyfish probably wasn't trying. It has no brain to try with. The thing that gets you might be drifting half-dead, or beached and drying on the sand, or just a scrap of tentacle torn off hours ago and floating on its own. A jellyfish doesn't decide to sting any more than a landmine decides to go off. Its tentacles are simply covered, end to end, in millions of microscopic loaded traps, and you brushed one.

Millions of tiny harpoons, waiting

Here's how jellyfish sting. Their tentacles are studded with specialized stinging cells called cnidocytes, and inside each one sits a nematocyst, a minuscule capsule holding a tightly coiled, barbed, venom-tipped thread, like a harpoon wound up under enormous tension (Ocean Conservancy). A single tentacle can carry thousands or even millions of them. When the trigger is tripped, the capsule fires: the coiled thread shoots out, spears whatever touched it, and pumps venom straight through the skin. Multiply that by the number that go off at once, and you have a sting.

What pulls the trigger

Each nematocyst has a tiny hair on the outside, a trigger called the cnidocil. Brush it and bend it, and that, combined with chemical signals from the surface of whatever's touching the tentacle, sets the cell off (Smithsonian Magazine). The firing itself is pure physics. The capsule is packed with such a high concentration of dissolved material that, the instant its lid pops open, water floods in by osmosis and the internal pressure rockets past 150 atmospheres. That pressure blasts the coiled thread out and turns it inside out as it goes, driving the barbed tip into the target like a spring-loaded needle.

One of the fastest things in all of biology

And it happens faster than almost anything else alive can move. When scientists filmed nematocysts discharging with cameras shooting more than a million frames per second, they found the harpoon launches in as little as 700 nanoseconds, under a millionth of a second, hitting accelerations of more than 5 million times gravity (Nüchter et al., Current Biology). The pressure at the tip rivals the impact of a bullet. The mass being thrown is almost nothing, about a billionth of a gram, but concentrate that much force onto that small a point and it punches clean through the armored shell of a crustacean.

The energy comes from the capsule wall itself, which sits stretched like a drawn bow. Trigger it and that stored tension releases all at once, a molecular spring uncoiling in nanoseconds. It is, by some measures, one of the fastest cellular events ever recorded, and the ocean is full of speed records like it, from the mantis shrimp's bullet-fast punch to this.

No brain required, and the venom is the variable

Because the whole system is automatic, the jellyfish never enters into it. There's no nervous system firing the shot, no choice, no aim. Each cell is its own self-contained weapon, loaded and waiting. That's the real reason the warnings about dead jellyfish and loose tentacles hold up: the traps don't need the animal to be alive, or even attached, to work.

What turns a harmless brush into a real emergency isn't the harpoon, which is much the same across species. It's the venom riding on it. Most jellyfish carry venom that does little more than leave you itchy and annoyed. A few, above all the box jellyfish, carry some of the most potent venom in the animal kingdom, strong enough to threaten a human life in minutes. Same delivery system, wildly different cargo. So the next time you give a stranded jellyfish a wide berth on the beach, you're respecting the right thing. It isn't the animal you're avoiding. It's the millions of fully loaded, hair-triggered harpoons it's still wearing, every one of them ready to fire the instant you touch down.

Keep wondering: the ocean keeps setting speed and survival records, from the fastest punch in the sea to the jellyfish that may have beaten death itself and the starfish that digests its prey outside its body.