40d:Flow

Flow is the direction in which liquids will move or "spill over" under the effects of gravity. Water and Magma are both liquids that are likely to be constantly on the move until either contained (such as in a Lake) or until they become far too stretched out. In the latter event, water will evaporate quickly, while magma will cool down and possibly leave behind Obsidian.

In the game world, most liquids are represented with a ≈ and ~ tile of appropriate color (blue, in this case, is water). Smaller amounts of liquids, such as Blood and Vomit, can be represented with other, smaller tiles.

Amounts of liquid are measured in sets of 7 layers per tile. You can see an exact amount of liquid per tile by switching your SHOW_FLOW_AMOUNTS to YES. A layer of 1 can be imagined as ankle-deep for a dwarf, while 7 layers is completely full. All liquids will spill into a nearby tile when there is more than one layer of liquid in a tile and the liquid is not contained.

Each type of liquid has their own properties, which means they're going to do their thing in different ways. It is reccomended you read up on individual liquids to see precisely what you can and cannot do with them. In addition, liquids do not exactly mimic real life conditions. Expect to see quirks such as a lack of consideration for air pressure in certain situations.