The sting of a jellyfish comes from specialized cells in the surface of its tentacles called cnidocytes. Each small, bulb-shaped cell holds a barbed, threadlike tube, called a nematocyst, filled with venom. On the outside of each cell is a tiny hair called a cnicocil. When this “hair trigger” is disturbed, the cell’s toxic harpoon explodes from its capsule and into the skin of the jellyfish’s prey, or an unlucky swimmer.
The amount and type of venom, and the effect that it causes, depends on the type of jellyfish, the number of nematocysts involved, and the area and thickness of the skin they strike. Whatever the variables, a sting is never exactly pleasant.
Often, after a human gets stung, there’s a tentacle or two that gets ripped off and left behind on their skin. The first step to treating the sting is removing the tentacles without triggering any unfired nematocysts and making things worse.
Pressure triggers the cells, so you can’t just pick them off (whoever is doing the picking is just going to get stung on the fingers, too). Certain chemical changes, like throwing off the salt balance between the outside and inside of the cell, can also can also cause the stingers to fire. This is why urine is often no good. Sure, urine contains salts, but it's just too variable. Concentrated urine might do the trick, and there are anecdotal reports from sting victims that it helps relieve some of the pain, but if the peeing rescuer is well-hydrated, the urine will be too diluted with water and make the stingers fire. What’s more, while urine is sterile, it has to pass through the germ-laden urethra to get out, and can lead to a bacterial infection of the sting wound.
What You Can Do Instead of Urinating on Your Friends
So, if Friends lied to us (gasp) and peeing on a sting runs the risk of making matters worse, what should you use to clean and treat the wound? Vinegar is usually the way to go -- just the plain white stuff with 5% acetic acid. It neutralizes unfired nematocysts so they can't sting anymore, and research (see here and here) has shown it to be one of the better post-sting rinses, especially when combined with the topical anaesthetic lidocaine.
When the one-two punch of vinegar and lidocaine isn’t available (and who brings vinegar on a swim?), sea water (the warmer, the better) is also good for rinsing away the remaining nematocysts. Once the stinging cells are deactivated, the stuck bits of tentacle can be picked off or scraped away with a credit card.
A word of caution, though: vinegar might not be the best treatment depending on the creature that stung you. Acetic acid can actually have the opposite of the intended effect on stings from the jellyfish lookalikes in the genus Physalia, like the Portuguese man o' war. If you’re not sure what stung you, stick with sea water or seek help from a lifeguard or medical professional.