Stung by a jellyfish at the beach, so a mate offers to pee on it, because urine neutralises the sting, as seen on Friends and repeated at every beach barbecue. The classic, slightly mortifying jellyfish remedy that everyone has heard.
Peeing on a jellyfish sting can make it worse, not better. A sting works because the jellyfish's tentacles leave behind tiny stinging capsules called nematocysts on your skin, some of which have not fired yet. Urine is a variable, unpredictable liquid, and depending on its makeup it can actually trigger those unfired capsules to discharge more venom, turning a bad sting into a worse one; plain fresh water can do the same thing. The general advice is to rinse the area to remove tentacles without setting off more stingers, then treat appropriately, and the right rinse depends on the species and region, which is why local lifeguard advice matters. For many stings, including the dangerous box jellyfish in northern Australia, the recommended treatment is dousing with vinegar, which disables the stingers, then seeking medical help for serious cases. So the friendly offer to pee on you is not just embarrassing. It is, on the evidence, the wrong move and may sting you twice.