Some still sting if their tentacles are wet. Tentacles torn off a jelly can sting, too. If you are stung, wash the wound with vinegar or rubbing alcohol. Or sprinkle meat tenderizer or put a baking soda and water paste on the sting. Don't rinse with water, which could release more poison.
Lifeguards usually give first aid for stings.
See a doctor if you have an allergic reaction. All jellies sting, but not all jellies have poison that hurts humans. Of the 2,000 species of jellyfish, only about 70 seriously harm or occasionally kill people.