Get some flint and steel, then right click the block you want to set fire on. Alternatively you can just hack in fire blocks and place them.
Chat with our AI personalities
You used to be able to hit it, but now you either have to make a redstone path or set the block next to it on fire.
The only way to keep a fire going indefinitely is to get a block of netherrack, if these are lit on fire, the block is neither consumed, nor extinguishes.
You get a three block deep down hole, fill the hole with lava, and drop the sword into it. This would destroy the sword though, which I'm sure you don't want. Other than this, you can also enchant a sword with fire aspect, which won't set the sword on fire but will set mobs and players on fire when they are hit with said sword.
Make a 4x5 obsidian frame, and set the middle of it on fire.
The hardest block in Minecraft is bedrock. It is completely unbreakable unless on creative mode.