Jellyfish are transparent and made up of 95 percent water, so you’d think there isn’t much to them. But you’d be wrong. Jellyfish are more complex than you’d think—and one of their most fascinating parts is their stinging cells. Located on their tentacles, jellyfish's stinging cells are called cnidocytes. They are small compartments that house a mini needle-like stinger. When an outside force triggers a stinger, the cell opens, letting ocean water rush in. This causes the stinger to shoot out into what triggered the action; once it’s there, venom is released. All of this happens within a millionth of a second. Though the venom of most jellyfish is not harmful, some can be deadly. For example, the Indo-Pacific box jellyfish—or sea wasp—releases venom that makes the heart contract. There is an antidote, but the poison acts fast, so someone who is stung must seek medical attention immediately.