Slime

A Slime is a hostile mob that spawns underground and in Swamp biomes.

Spawning
Slimes spawn in specific chunks below layer 40. They can also spawn in swamp biomes between layers 50-70 in light levels of 7 or less.

Like other hostile mobs, slimes will not spawn within 24 meters (spherical) of any player, and will despawn over time if no player is within 32 meters and instantly if no player is within 128 meters.

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.

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 which 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 layer 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 +                       (long) (xPosition * xPosition * 0x4c1906) +                        (long) (xPosition * 0x5ac0db) +                         (long) (zPosition * zPosition) * 0x4307a7L +                        (long) (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 will be generated, and if that number is 0, the chunk will be able to spawn slimes. To convert world coordinates to chunk coordinates, divide by 16 and round down.

Like most other mobs, Slimes require two vertical non-opaque blocks (e.g., air, signs, torches) 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 3x2.5x3 space to spawn, small slimes require a 3x2x3 space, and tiny slimes require a 1x2x1 space (or 1x1x1 if the upper block is not opaque). Slimes are equally likely to spawn at all sizes. Slimes are much more likely to spawn in a 3 high room.

Drops
Slimes drop experience points equal to their size.

If a slime's size is 1, it will also drop 0-2 slimeballs.

Behavior
Only sizes 1, 2, and 4 spawn naturally, and with equal probability. With use of, slimes can potentially range from size 1 to 256.

Slimes move by hopping, which they will do each 10 to 30 ticks ($1⁄2$ to $1 1⁄2$ seconds). Their exact routine is as follows:

The Slime will search for a player within 16 blocks (spherical) distance.
 * If no players are found, they wait 10 to 30 ticks ($1/2$ to $1 1/2$ seconds). Then they will change direction, by a random amount up to 57.26° (1 radian) left or right, jump, and repeat the process.
 * If a player is found, the delay before jumping will be $1⁄3$ as long (3 to 10 ticks), and the Slime's direction will be set directly toward the nearest player before jumping.

A slime's health is equal to their size squared, and its dimensions are 0.6 blocks times its size in each dimension. When a slime attacks, it will deal damage equal to its size, except for size 1 (smallest) slimes, which cannot do damage.

A slime's jump distance also depends on their size; a slime will jump a distance slightly farther than its length.

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

Slimes also have an odd attack interval: that is, their attack speed is twice that of other melee-combat mobs. 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.

Pre-1.4 spawning
The (pre-1.4) slime spawning algorithm is discussed in greater detail in this forum topic, but note that the algorithm has changed many times and older information may no longer be accurate.

Trivia

 * Slimes do not prevent you from sleeping.
 * Slimes will only attack the player, even if a skeleton accidentally shoots it.
 * Slimes hop much faster when pursuing a player.
 * From one large slime, the maximum experience a player can get from killing it and all the slimes that split from it is 28 XP; 4 XP from the largest, 8 XP from the 4 it splits into, and 16 XP from the 4 each of those split into.