Minecraft wiki entry for Exploring_caves, associated text: (Water) buckets - Empty buckets stack (while water or lava buckets don't), but the player will want to keep at least one bucket already filled with water. This is useful for turning lava into obsidian, allowing it to be safely walked on or mined. Note that there can still be lava below it though, and the water should keep flowing. This also useful for extinguishing themselves when on fire. Empty buckets can let the player move water or lava springs out of their way, or collect more water or lava for their own purposes. Water buckets can also function as a ladder, and are needed to capture mobs like the tropical fish and axolotl. Lava buckets are also useful for fighting mobs, especally for slimes.