Fire

Fire is a block that was first seen in Indev. It has an animated face on all four sides, and two faces on the inside at slants. Fire is not created naturally in a map, it will only be created if the player causes it or Lava burns a flammable object. Fire produces light.

Fire is only available for use in Indev, Infdev and Alpha. It cannot be directly placed (although it could be in an old Indev version designed to test out fire), but rather by using Flint and Steel. When placed, the fire will burn for a short, randomly determined period of time. If nothing flammable is next to it, it will die. Planks, Trunks, Leaves, Cloth and Bookcase blocks are all flammable, while TNT will detonate if exposed. 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.

As one would expect, any mob (including the player) will be lit on fire if exposed to it. The fire will obstruct the player's view slightly and they will slowly lose life. The fire will burn out after a while, but it is a good idea to try and jump into some Water as soon as possible. Fire can be used as an attack against mobs. Lighting 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, mob or dropped object, will be lit on fire.

Punching or hitting an object once will extinguish the fire on the side that was hit. Water will also extinguish fire.

Eternal Fire
In Alpha, a 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.

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

PCGamer's article reveals that the dirt from The Slip will burn infinitely.

Chainmail Armor
Due to a bug, 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 hacking it.

See this video.

Eternally Burning Trees
Due to a bug, if a Flint and Steel is used to light a tree on fire, it becomes possible for pieces of the Trunk to burn eternally. See "Eternal Fire" above. You can create this effect by extinguishing (punching) the fire on the sides of the trunk but leaving the top alight.

As a Trap or Fuse
Using eternally burning fires can be effective as cheap mob traps/defenses, but will sometimes burn the loot. Also, since fires generally spread slowly, they can be used as a fuse to set off TNT or to release water/lava, though this is unreliable since fire does not always travel predictably.