POUR

Syntax: pour <from container> <to container>
fill <to container>

The fill command is used to fill a liquid container, such as a waterskin.
You can only fill something if there is a fountain or other water source,
such as a stream or lake, nearby.

Pour will transfer liquid from one container to another. Both containers
must be in your inventory for it to work.
You may also use pour <container> out  to empty it without pouring the
contents into another container.

Example:
fill canteen
pour keg mug