Slime

Slimes are cube-shaped hostile mobs that spawn deep underground in particular chunks, aboveground in swamps, or when an Oozing-affected entity dies. They move by jumping, chasing after their targets, and usually dealing damage on touch. Slimes spawn in different sizes, and larger ones split into smaller ones on death. Slimes are a major source of slimeballs.

Slimes spawn naturally in 3 sizes: small, medium, and large. A fourth size, known as semi-large, only spawns from trial spawners and the oozing mob effect[verify]. Slimes have an NBT tag, [Byte] Size, which determines their size. Small, medium, and big slimes have their Size set to 0, 1, and 3 respectively. Using the /summon or /data commands, a slime can be given any Size in the range from 0 to 126 (inclusive).[verify for Bedrock Edition]

A slime needs a space of 2.04×2.04×2.04 blocks to spawn, which must be clear of solid and liquid obstructions. When a slime attempts to spawn, the game checks to see if the space requirement is followed. Therefore, small and medium slimes are not able to spawn in 2-block tall areas even though it would normally be enough room for other small mobs. Any block within the space, even a glass pane, can prevent slimes from spawning.