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The Flint and Steel will create a fire on the targeted surface of a block when used. Some blocks such as wood and wool will burn down when set on fire, but some blocks such as stone , will not be affected. Interestingly, any use on an item will reduce its Durability by one, regardless of whether it is set on fire or not. If used on the top of a non-flammable block like stone, it will set a fire for about a minute and then it will burn out, but if used on the side of the block the fire will burn out immediately. If you place fire near flammable blocks, like wood, it will spread to the flammable blocks, but it will not spread to Netherrack . Flint and steel was also a very infamous griefing tool back before beta 1.6 when fire spread indefinitely, and is still somewhat dangerous.
Flint and Steel can be used to light blocks on fire, activate TNT , or activate Nether Portals. However, Flint and Steel is an infamous griefing tool, explaining why so many beta servers ban Flint and Steel. When a Flint and Steel is placed within a dispenser , when activated, it will set fire to the adjacent block.
Also, Flint and Steel can be used as a weapon. If being chased by a player in Multiplayer PVP, for example, flint and steel can be used to light the player on fire, thus damaging their health.