40d:Fire

A border of exclamation marks (!!) on an item or dwarf name indicates that it is on fire. Items on fire also release smoke and ignite adjacent items. Burning items will eventually vanish, being completely consumed by the fire. The color of the name of burning dwarves will alternate between yellow and red (it may do so forever after the dwarf died).

Dangers

Fire poses special problems in that burning items and dwarves set other nearby items on fire. A good example:

Dwarf gets hit by fireball from fire imp at the magma forge
Dwarf runs around his business, as he takes burning damage he goes for a rest, and heads for the barracks
The dwarf dies on the way. Someone comes and picks up his !!giant cave spider silk shoe!! and puts it on. His clothing catches on fire
This dwarf also feels hurt and heads for the barracks, but this one gets on the wooden bed, lighting that up.
Nearby beds light up; dwarves sleep on that bed and the others near it and get lit up themselves, as well as the dwarves going for the !!cave spider silk robe!!
Some of these dwarves go past the alcohol stores for a drink, these light up. now a fair portion of the fortress is on fire.
Remaining dwarves will have other barracks to sleep in, and some of them have rooms. they go back and set more of their stuff on fire
Three months later: everything is burning and all your dwarves are dead because none of them fell in the river, putting the fire out.^[Verify]
Wasn't that fun?

Forest fires

Origins

Creatures such as Fire imps can cause a forest fire by igniting a creature with a fire attack. If grass also ignites, the fire will slowly spread to all connected grass, generating smoke as it burns. Fire turns grass into ashes, but not usable ashes. Anything that is flammable and stands on burning grass (if you look at it you will see "A fire") has a chance to catch fire each turn.

Preventing forest fires

It is recommended to guard the surroundings of an exterior magma pool with Marksdwarves so that creatures don't have a chance to light anything up (except the Marksdwarves themselves, but this is unlikely if they are well trained). Having walls or better yet fortifications all around the pools is likely to prevent all possible forest fires. Strangely grizzly bears seem to be ineffective against such a threat.

Alternatively, pour water on top of the pool to turn it to obsidian, sealing it. (But what a waste of magma!)

Protecting from forest fires

Since dwarves will not pay attention to fires , and dwarves will rush to move their dead friends to the graveyard, you may end up very quickly with heavy casualties if you do not order all dwarves to stay inside. If you have precious items and buildings outside, it is also possible to dig a channel to stop the fire from spreading any more (they are cheaper then walls, offer the same protection and, most importantly, are much faster to create).

Know however that merchants will not pay anymore attention to the fire than your own dwarves, so you may also want to protect them in the same fashion, even if it should change their path, otherwise they could all happily walk into flames when leaving the region (or worse, stand for days on "A fire").

Irregularities

Forest fires present certain irregularities that you should take in consideration ^{v0.28.181.40d}:

Trees, plants and other map tiles will not be destroyed in the fire. That makes them very easy to spot after a forest fire, and also a possible (if unlikely) natural defense. Items, however, may burn.
Grassy slopes will not catch on fire, thus preventing the fire from spreading across z-levels.
Forest fires, no matter how big, will never cross a channel or a non-wooden road. However, they will go through closed doors. ^[Verify]
In you start a fire in adventure mode, it will not spread if temperature is turned off.

Use as a weapon

This is not recommended, although some success has been reported with the Booze Bomb trap (which consists of a fire imp in a cage linked to a lever, standing near an alcohol stockpile so that it can be released when the enemy get near). It should be noted that this is a criminal waste of good liquor (unless you use crap like Prickle Berry Wine, Sewer Brew or Gutter Cruor..).

Adventure Mode

In adventurer mode, it's possible to ignite trees and shrubs with g. The possibilities for killing wolves with the burning corpses of other wolves should not be overlooked. (It should be noted that the Travel function puts out any inconvenient fires on yourself and your collection of "!!wolf head!!"s, which will probably end up as "Xwolf headX" after your trip). Since steel items do not burn, it is possible to kill friendly guards by setting the right location on fire, and when the hapless speardwarf perishes, heading back and taking his spear to use as a throwing weapon.