Anemones have tentacles that sting, but the clownfish isn't bothered by them. In fact, it lives among the tentacles.
Living among the tentacles of the anemone, the clown anemonefish gains protection from predators—which don't dare get near the stinging protector. The anemonefish also gets to eat leftovers from the anemone's meals. The anemone benefits from having the fish around, too. The useful fish nibbles away parasites that bug the anemone. And sometimes the anemonefish brings food into the tentacles which the anemone can also eat. Generally several anemonefish make one host anemone their home.