Fire

Fire is a block. It has an animated face on all four sides, and two faces on the inside at slants. Fire is never created naturally when chunks generate, except in the Nether. It will be created if the player or a Ghast creates it, when Lava burns a flammable object, if lightning strikes a flammable block or by using the Flint and Steel tool. Fire produces 15 units of light.

Behavior
Fire is not available for use in Classic mode. Flint and Steel is one method used to place Fire in the environment. When placed, fire will burn for a short, and randomly determined period of time. If nothing flammable is adjacent to it, the flames will not spread, and will die out. Wood, Logs, Leaves, Vines, Wool, and Bookcase blocks are all flammable; and TNT will detonate if exposed to fire. Fire burns leaves slightly faster than the other flammable objects.

Fire will spread over flammable surfaces, slowly causing all the blocks it can reach to burn away. Fire can climb up walls, across floors and ceilings and over small gaps. More precisely, a fire block can turn any air block into fire that is adjacent to a flammable block and up to one block downwards, one block sideways (including diagonals), and four blocks upwards.

As one would expect, any mob and players will be lit on fire if exposed to it. Fire will obstruct the player's view slightly and they will slowly lose life at a rate of 1/2 heart per second. This is the same rate that the player gains health in peaceful mode, so fire will not kill you in peaceful. Fire will burn out after a while, but it is a good idea to try to jump into nearby Water as soon as possible. A bucket filled with water can function as a portable fire extinguisher as a player who is on fire can dump the water at their feet to put out the fire instantly.

Fire can be used as an attack against mobs. Igniting dropped items or blocks on fire will cause them to burn away. This trick can be used to dispose of materials quickly. Anything that touches Lava or Fire, whether it is a mob or a dropped object, will be lit on fire. Although lava can set off TNT, it takes several minutes and is easily fixable if the lava can be reached and cleared.

Cows, pigs and chickens will drop cooked meat if they die while they are on fire. However, while attempting to burn them with lava, they will sometimes drop raw meat if the damage tick from the lava was the decisive one to the animal's life. This is because the burning attribute for Lava only comes after the damage which the Lava block deals upon contact, so the animal will die from normal damage without being burnt.

Punching or hitting the side the fire is on will extinguish the fire on the side that was hit. Hitting fire with a tool does not use up any uses for the tool. Placing Water, sand, and gravel will also extinguish fire.

Eternal Fire
Netherrack, a block found in The Nether, will burn forever when lit on fire. It can therefore be a useful trap or defense material around the player's shelter; though the player should be reminded that everything will burn in fire, including dropped items that fall in fire.

Lit Netherrack can also be used as an alternative to torches. If the player is running low on coal then they can use Netherrack and Flint and Steel instead. It is brighter than torches and can be quickly extinguished. Note that rain won't put out Netherrack fire.

Due to a bug, the fire may sometimes not emit any light.

Chainmail Armor
It is possible to craft Chainmail Armor out of fire blocks. It is only possible to get actual fire blocks in the players inventory by means of using the  command or an Inventory editor. Chainmail Armor is crafted like any other armor and has the same durability as Gold.

History
When introduced in Indev Fire could be directly placed before Flint and Steel were added. Before furnaces were added, fire was used to smelt objects.

Since Alpha version 1.2.6, forest fires may start spontaneously if an above-ground lava pool is generated amongst trees during a biome's creation. This is a rare side-effect of starting a new world or exploring.

In versions from Alpha to Beta 1.2_02, a non-netherrack flammable block, like wood, may burn continuously when its sides are surrounded by non-flammable blocks, like stone or dirt (the bottom is not necessary, though it is best not to have anything explicitly flammable underneath). A fire will only stay burning at the top of a flammable block, not on the sides or the bottom. Once the fire is burning - and has continued to do so for ten seconds or more without consuming the block beneath - you may remove all the adjacent blocks, and it will keep going forever. If the block burns away, simply replace and re-ignite until it works. Alternatively, you can punch out any adjacent fires that appear instead of surrounding the block. Eternally burning blocks were fixed in Beta 1.3.

It was also possible to make larger blocks of endless fire by putting flammable blocks diagonally adjacent to a block that is burning continuously. Since the already-burning block counts as non-flammable - it is already on fire - the adjacent blocks may also burn continuously. By igniting multiple blocks this way, you could make a grid of diagonally adjacent burning blocks.

Notch has stated that he has disabled the infinite spreading of fire, which is now in the 1.6 update. Notch's Twitter

Trivia

 * When smoke hits water, it hisses just as if water has hit a fire.
 * It is possible for lightning to light a surface on fire during thunderstorms, but it will almost immediately be put out by rain. In rare cases in SMP, fire may not get extinguished.
 * The texture for fire is not obtained from terrain.png; instead, the fire animation is calculated by code and drawn into the "FIRE TEX" area of the in-memory copy of terrain.png. Water, lava, and portal textures work similarly. This is why >16×16 texture packs have glitches; the area drawn on is not scaled to fit the larger texture.
 * While wood blocks are flammable, two wooden half blocks on top of each other are not. This can be used to make fireproof wooden buildings, especially in the Nether.
 * Though wood planks and logs are flammable, they burn up very quickly, making them virtually useless when making a fireplace, bonfire, etc. The only block that burns forever is netherrack.
 * On very laggy computers, set a fire on the side of a block, then log off, and then log back onto the world. In a very laggy moment, you will see "FIRE TEX HINT!" instead of the fire animation.
 * Entities such as Mobs or Items on fire do not emit any light.
 * Putting the actual fire block in the inventory, chest, or dispenser, and update Minecraft, the name will show "Undefined item" and when clicked, the game crashes.

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