JellyfishA jellyfish has long nematocysts with poison. Do not rub or scratch. If you try to wipe away the mucus, you are re-injected with poison. So this only creates more pain.
Use rubbing alcohol, or as an aperitif alternative liquor or vinegar for at least 30 seconds. This will inactivate the nematocysts. (First test on a part of skin to see if it is tolerated by the patient.) Otherwise, use salt water (not in the case of box jellyfish), no fresh water.) Afterwards the tentacles can be removed with water, sand or scrape.
Wear best (disposable) gloves to remove the tentacles carefully.