“As Moses and the children of Israel were crossing the Red Sea, the children of Israel began to complain to Moses how thirsty they were after walking so far. Unfortunately, they were not able to drink from the walls of water on either side of them, as they were made up of salt water. A fish from the wall of water heard the complaints and told Moses that he and his family could remove the salt from the water through their own gills and force it out of their mouths like a fresh water fountain. Moses accepted this kindly fish's offer. But before the fish and his family began to help, they told Moses they had a demand. They and their descendants insisted that they always be present at the Seder meal, since they had such an important part in the story. When Moses agreed, he gave them their name, for he said to them, "Go Filter, Fish!"

haggadah Section: Introduction
Source: An Old Jewish Joke