Bucket

Buckets are tools used to carry water, lava, mud, milk and fish.

Usage
Empty buckets can be used to "pick up" a water, lava or mud source block by pressing item on a block of the relevant type.

Similarly one can press when looking at a cow to fill a bucket with milk.

Empty buckets can also be used to collect fish in buckets.

A bucket filled with a source block can then be used to place its source block contents in the empty block next to the side of the block the player is currently looking at, or replacing the block looked at for some replaceable blocks.

A bucket full of milk can only be emptied by drinking it or by using it in crafting a recipe (like cake).

Placing an empty bucket in the fuel slot when smelting wet sponge in furnaces will fill it with water after the process.

An empty bucket can be used to empty a cauldron with water, filling the bucket with water.

An empty bucket can be used to empty a cauldron with lava, filling the bucket with lava.

Data values
In Bedrock Edition, buckets use data values to determine contents. Once used, each data value is set to 0 (Empty Bucket). If no data value is provided, the data value is 0.