Faucets can be used to extract fluids from a container into the one below.

Faucets can be placed on any side of a block, or on the bottom. They will always flow towards the block below

Faucets can be activated from either a redstone pulse or right click. If a faucet is pouring, it will pour until it either receives another pulse or right click, or the tank below is full.

Faucets are the primary way to fill channels.