Slime

Slimes are bouncy cube-shaped hostile mobs that spawn in the swamp, and occasionally, deep underground.

Spawning
Slimes spawn in the Overworld in specific "slime chunks" below layer 40 regardless of light levels. They can also spawn in swamp biomes between layers 50 and 70 in light levels of 7 or less.

Only sizes 1, 2, and 4 spawn naturally. With use of, slimes can potentially range from size 1 to 256 (NBT  tag 0–255).

Slimes do not spawn within 24 blocks (spherical) of any player, and despawn over time if no player is within 32 blocks and instantly if no player is within 128 blocks.

Slimes require two vertical transparent blocks (such as air, signs, or torches, but excluding redstone-related items) to spawn in, with an opaque block underneath. The space they spawn in must also be clear of solid obstructions and liquids. Big slimes require a 3×2.5×3 space to spawn, medium slimes require a 3×2×3 space, and small slimes require a 1×2×1 space (or 1×1×1 if the upper block is not opaque).

Slime size is affected by regional difficulty: chances range from 33% for each size at low difficulty to 16% small, 33% medium, and 50% big with higher difficulty.

Swamps
In swamps, slimes may spawn at night between the heights of 50 and 70 provided the light level is 7 or less. They spawn most often on a full moon, and never on a new moon. Slimes spawn in the normal swamp, and not in its variant, swamp hills.

More precisely, the game checks if the light level is equal to or less than a random integer (from 0 to 7), then if the fraction of the moon that is bright is greater than a random number (from 0 to 1). If these conditions are met and the altitude is acceptable, there is a 50% chance of spawning a slime.

"Slime chunks"
Slimes spawn throughout the world (except mushroom islands) below level Y=40 regardless of light level, but only in certain chunks, 1 in 10 of all chunks. These "slime chunks" are determined pseudo-randomly by combining their chunk coordinates with the seed of the world: Random rnd = new Random(   seed +    (int) (xPosition * xPosition * 0x4c1906) +    (int) (xPosition * 0x5ac0db) +     (int) (zPosition * zPosition) * 0x4307a7L +    (int) (zPosition * 0x5f24f) ^ 0x3ad8025f ); return rnd.nextInt(10) == 0; That is, using the chunk coordinates to help generate a seed, a random number between 0 and 9 inclusive is generated, and if that number is 0, the chunk can spawn slimes. To convert world coordinates to chunk coordinates, divide by 16 and round down. Note that are 32-bit integers ( int s).

Bedrock Edition
The slime chunk algorithm $$ is different from $$. The algorithm doesn't depend on the world seed, thus the chunks that slimes can naturally spawn in are the same for every world.

Drops
If a slime's size is 1, it drops 0-2 slimeballs. This can be increased by 1 per level of Looting, for a maximum of 5 slimeballs.

Slimes also drop experience points equal to their size.

Behavior
Slimes move by hopping, which they do each 10 to 30 ticks ($1⁄2$ to $1 1⁄2$ seconds), and can swim in water and climb ladders. Unlike other mobs, slimes continue moving when no players are nearby. Their exact routine is as follows:

The slime searches for a player (or, failing that, an iron golem) within 16 blocks (spherical) distance. They can see even through blocks.
 * If no target is found, the slime waits 10 to 30 ticks ($1/2$ to $1 1/2$ seconds). Then it changes direction, by a random amount up to 57.26° (1 radian) left or right, jump, and repeat the process.
 * If a target is found, the delay before jumping is $1⁄3$ as long (3 to 10 ticks), and the slime's direction is set directly toward the target before jumping.

A slime's health is equal to its size squared, and its dimensions are 0.51 blocks times its size in each dimension. When a slime attacks, it deals damage equal to its size, except for size 1 (smallest) slimes. Small slimes do not prevent sleeping.

A slime's jump distance also depends on its size; a slime jumps a distance slightly farther than its length. When landing, an amount of slime particles 1/8th its size appear.

When the player kills a slime and the slime's size is larger than 1, it dies and spawns 2-4 new slimes equivalent to its size divided by 2, rounding down.

A slime's attack speed is twice that of other melee-combat mobs. This speed is noticeable when the slime has the player cornered at a wall. Mobs like zombies and spiders attack at a rate of one hit per second, while slimes attack at a rate of two hits per second. Slimes damage all players and iron golems they collide with, unlike other mobs that damage only those targets they specifically attack.

Data values
Slimes have entity data associated with them that contain various properties of the mob.

Trivia

 * From one large slime, the maximum experience a player can get from killing it and all the slimes that split from it is 28 experience; 4 experience from the largest, 8 experience from the 4 it splits into and 16 experience from the 4 each of those split into.
 * A named large slime that dies produces smaller slimes with the same name.
 * $$, with commands, the sizes of slimes can be customized. Sizes go from 1 up to 256; the size-256 slimes the largest of any mob in the game, bigger than the ender dragon.
 * Small slimes are the weakest of all mobs: they have only 1 health point and deal no damage, despite being a hostile mob.
 * Because a slime's movement speed is tied to its size it becomes impossible for the player to outrun a size 8 slime on flat ground without potion effects.
 * When a slime is searching for nearby players for the purposes of attacking them or checking to see if it should despawn, it checks not from its outer edges, but instead from a point in the center of its hitbox on the x and z-axis and the bottom of its y-axis. This means when a slime is spawned with a huge custom size it can be right in front of the player and be completely passive, it may even despawn despite its outer edge is less than a block away if the slime is large enough.
 * Never use a shield to combat slimes. An entire shield can be wiped out of durability within 10 seconds without the Unbreaking enchantent.