Animals in the phylum Cnidaria, which means “stinging nettle,” possess stinging cells called nematocysts. Cnidaria includes jellies, box jellies, sea anemones, corals, sea pens and hydroids. Nematocysts are cells that operate like a combination between a spring and a harpoon. A spring with a barbed tip is wound underneath a trap door covering called an operculum. When properly stimulated, this door opens, allowing the poison-coated barbed tip to spiral out towards its target. Only certain species of Cnidarians have springs and barbs strong enough to penetrate human skin, which results in the commonly recognized jelly sting.
The well-known folk remedy for a Cnidarian sting is urine. However, urine has the wrong chemical composition for inactivating the nematocyst, so this method does nothing but supply embarrassment to the involved parties. According to the American Red Cross, the best way to deactivate the stinging cells when stung is to rinse the area with vinegar for 30 seconds. Then, according to the American Heart Association, the best way to relieve the pain of a sting once deactivated is to rinse or soak the area in hot water for 20 minutes.