Jellyfish sting through specialized cells called nematocysts (or cnidocytes) located mainly on their tentacles. Each nematocyst contains a tiny, coiled, barbed harpoon-like tubule under high pressure, filled with venom

. When something brushes against a jellyfish's tentacle, tiny triggers on the nematocyst's surface activate it. This causes the cell to open, letting seawater rush in, which rapidly increases pressure inside the nematocyst. The pressure forces the barbed tubule to shoot out in less than a millionth of a second, piercing the skin and injecting venom into the victim

. This venom can cause immediate pain, burning, itching, swelling, and red welts on the skin. In some cases, especially with more dangerous species like box jellyfish or Portuguese man-of-war, the venom can cause severe systemic reactions affecting muscles, breathing, or even the heart

. Notably, nematocysts can still fire and inject venom even if the jellyfish is dead or detached from its tentacles, so touching jellyfish remains or tentacles on the beach can still cause stings

. In summary, jellyfish sting by firing microscopic venomous harpoons from nematocysts on their tentacles when triggered by contact, injecting venom that immobilizes prey or deters threats