Falling Block

A falling block is the entity form of block that appears when a gravity-affected block loses its support.

Spawning
Falling blocks naturally spawn where gravity-affected blocks lose their base of support. These blocks include:

Although the same appearance as its corresponding block and affected by gravity, the primed TNT entity is not a falling block.

If cheats are enabled, falling block entities can be spawned using the command. By tweaking the  NBT tag, it is possible to summon a falling block entity that normally does not fall as a block, such as dirt or stone, although it will have the same behavior as a naturally spawned falling block.

Behavior
A falling block continues falling until it lands on another block with a solid top surface. After landing, if the block in its place (the coordinates of the center at the bottom of its hitbox) is replaceable (grass, water, vines, air, etc.) then the falling block returns to its block state. Otherwise, it breaks and drops as an item.

Like most other entities, falling blocks are affected by explosions and bubble columns, can be launched by a moving slime block (although they do not bounce on stationary slime blocks), can be pushed by pistons, slowed down by cobwebs, and can slide down the side of a honey block. However, they can neither be pushed nor slowed down by water or lava.

Falling blocks do not have health, cannot be attacked and do not take damage. They are also immune to all status effects. They can be removed with the command.

Commands such as can change the moving and facing directions of a falling block, although there are no in-game mechanics that may alter a falling block's facing direction.

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