The skipper of an aluminium boat was in the water scraping barnacles off the hull when water started rushing into the boat. The bilge pumps started to work, and the crew frantically searched for the leak, which couldn’t be found. Suddenly the bilge pumps started to win, and the water level in the boat dropped. The bilges dried out, and no more water was leaking into the boat. The skipper and the crew spent the night worrying about what had happened, but there were no more leaks. The next day the skipper went back to cleaning the hull. This time, as well as barnacles, he found a big jellyfish stuck to the side of the boat, which he also scraped off. He heard the bilge pumps start to work, and again there was water pouring into the boat. This time the frantic crew eventually found a neat circular hole beside the toilet, which they plugged with a wooden bung.
A few weeks later, still puzzled about the hole, the skipper was relating the story to an experienced old sea dog, who asked him whether anyone had lost a watch battery recently. Strangely, one of the crew had been sitting on the toilet some weeks earlier changing their watch battery when they had dropped it and it had rolled away and disappeared. The sea dog laughed and gave his explanation: the watch battery had rested against the aluminium hull, causing it to corrode and making a nice neat hole directly under a big barnacle which plugged the hole until… it was scraped off by the skipper, causing water to rush into the hole sucking in a big jelly fish which again plugged the hole until… it was scraped off by the skipper the following morning!