Fluid
Water drops one level per block, so it reaches seven, and it looks four blocks ahead for a hole to fall down.
Draw
Click and drag on the plan. Ground is drawn as a height map, pale for high and dark for low, and a yellow outline marks a source.
Start from
Two sources with one gap between them make a third, which is the infinite water pool. Lava never does that in any dimension.
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.
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.
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.
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.
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.
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.
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