Slimes
The slime is the fifth hostile mob, added in Minecraft Alpha 1.0.11. They appear as green semi-transparent cubes. Slimes come in four official sizes: huge, big, normal, and small. When a player attacks a huge, big or normal-sized slime, it will break off into four smaller sized slimes (huge to big, big to normal, normal to small) unless a sword is used. Hurt slimes will not grow larger over time. Slimes of any size will burn instantly upon touching lava and die moments later without splitting into smaller slimes. Only huge, big and normal-size slimes can deal damage; small ones are harmless. Slimes are usually found in deep caves. This is partially caused by the fact that they will not spawn more than 15 squares above the bottom of the map (10 squares before the topmost bedrock). They are currently available in all difficulties, even in peaceful where small slimes will spawn. Note, however, that a small slime can grow larger over time, turning it into normal sized slime, thus making it the only mob that can attack you in peaceful. This may be largely due to a bug causing slimes to not be removed when switching to peaceful. Huge slimes are currently the largest hostile mob in the game. Slimes are the first ever hostile mob that can intentionally spawn in light and dark areas as they are regional (they only spawn within 15 squares of the bottom of the map).