This idea has been around a long time, and even made its way onto an episode of the popular TV show Friends. However, we can say without hesitation that applying urine to a jellyfish sting isn’t going to help much (plus it’s pretty inconvenient at a public beach).
Salt water, however, is a good way to reduce the pain of a jellyfish sting (don’t use fresh water or rub the area, as this can make it worse). Bits of remaining tentacles in the skin can be carefully removed by scraping with a credit card or something similar. The pain of the sting should gradually subside over the next 24 hours. If the sting is severe and covers a large area, or causes nausea, vomiting, difficulty breathing, or pain in an unaffected area, seek medical attention immediately. A few species of jellyfish are very toxic and their stings can be dangerous.