Dimension
Water drops one level per block, so it reaches seven and looks four blocks ahead for a hole. Overworld lava drops two and looks two, so it only reaches three.
Draw
Click and drag on the plan. Ground is a height map, pale for high and dark for low. A blue outline marks a water source and an orange one marks lava.
Start from
Water touching lava turns it to stone: obsidian where the lava was a source, cobblestone where it was flowing, and plain stone where lava fell into water. That is a generator in one rule.
The rule that surprises people
Fluid does not spread evenly and then find the hole. It finds the hole first. The game searches four blocks ahead in every direction, then flows only along the shortest path it found, which is why one badly placed gap drains a whole channel the wrong way.
That search is the thing to design around. A farm that relies on water reaching the far corner will fail the moment somebody digs a hole nearer to the source, and a moat with an unnoticed one block drop will empty into it rather than going round. Draw the ground here first and you will see it before you build it.
Generators
Two of the presets are the builds everyone makes. The cobblestone generator runs water and lava at each other along a channel, and the grey blocks appear where the flowing lava is caught. The obsidian one puts water directly over a lava source, which is the only way to get obsidian without a bucket. Both are the same rule seen from different angles, and the readout under the view counts what came out.
What this does and does not copy
The spreading rules are a port of the game's own FlowingFluid: the level drop, the slope search, falling columns restarting at full strength, and the two source rule. What it does not copy is the order of arrival. Minecraft schedules an update per block, five game ticks apart for water and thirty for Overworld lava, so real water crawls outwards in a slightly ragged front. This sweeps the whole grid at once, which reaches the same resting state and gets there in fewer steps.
Frequently Asked Questions
How far does water spread in Minecraft?
Seven blocks from a source on flat ground. Each block travelled costs one level out of the eight a source holds, so the eighth block gets nothing. Water falling down a hole restarts at full strength when it lands, which is why a waterfall spreads another seven blocks at the bottom.
How far does lava spread?
Three blocks in the Overworld and seven in the Nether. Lava costs two levels per block instead of one everywhere except the Nether, where it behaves exactly like water. That is the whole difference, and it is why a Nether lava spill is so much worse.
Why does water flow towards a hole instead of spreading evenly?
Because it looks ahead. Before spreading, the game measures how many blocks each direction is from somewhere the fluid could fall, searching up to four blocks for water and two for Overworld lava, and it only flows the way or ways that tie for the shortest. A single hole three blocks away will take the entire flow and leave the other three directions dry.
How do I make an infinite water source?
Two source blocks with one square between them, over something solid. When a block has two sources beside it and support underneath, it becomes a source itself, and it does so again every time you take it. Lava has that rule switched off in every dimension, so there is no infinite lava.
What decides whether I get cobblestone, stone or obsidian?
Geometry, and only geometry. A lava block with water above it or beside it turns to stone the moment they touch: obsidian if that lava was a source block, cobblestone if it was flowing. Lava falling down into water makes plain stone instead. That is the whole rule, and it is why a generator that drops the lava rather than running it sideways gives you the wrong block.
Why did my cobblestone generator make obsidian?
Because the water reached the lava source rather than the flowing lava a block or two along from it. Move the water so it meets the lava after it has spread, or move the lava source further back, and the same layout gives cobblestone. Build it here first and the two colours tell you before you place a single block.
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