Minecraft wiki entry for Cauldron, associated text: A cauldron can be filled with water by using a water bucket on the cauldron. Once completely filled, a cauldron can be used to fill a water bucket by using an empty bucket on the cauldron; this empties the cauldron. Despite containing water, using a fish bucket on a cauldron does not fill it with water, but places water against it, meaning it does nothing to the cauldron[1]