This is an old folk tale, which I think originates from Eastern Europe but like most folk tales, there are slightly different versions, which makes it all the more interesting. The basic plot is about a traveller/a group of travellers (some versions tell the tale with a solider, another with a monk, and others with animals rather than humans as the characters) who arrive/s in a village with an empty cooking pot. No one is willing to give or share food. So, they drop a stone into their pot and begin to boil it with water. Each villager asks what’s being cooked and is told ‘stone soup’ and that everyone can taste it. Each villager then brings a vegetable or herb to make it taste better so by the end, the stone is removed, and there really is food for everyone. It may not strictly speaking be a potion but it’s a tasty concoction and a clever way to show how the villagers had to be tricked into doing the right thing.