Jellyfish sting in order to get food.  A jellyfish is a simple, primitive sea animal with a jelly-like body.  It eats other small creatures of the sea.  The main part of the jellyfish’s body looks like an umbrella.

Hanging down from the umbrella are string-like tentacles. The tentacles are armed with stinging cells that contain a paralyzing poison.

When a fish or other small sea animal brushes against the jellyfish, the jellyfish paralyzes the animal with its sting.  Then the tentacles pull the victim up to the jellyfish’s mouth, which is at the bottom of the umbrella.–Dick Rogers