Short answer
Water is a critical Sandustry resource used to wet Sand and Seeds. The official Wiki lists surface lakes, underground deposits, melting Ice, Steam that reaches the sky and returns as rain, and Lumlings as sources. Water can flow through many building spaces but is blocked by specific structures, and its interaction with Sand creates Wet Sand for processing.
What Water does
When a Water pixel touches a Sand pixel, both are replaced by Wet Sand according to the official mechanic. When Water touches a Seed, the Water is absorbed and the Seed becomes Wet Seed. Those two reactions explain why Water belongs near the beginning of many production chains: it changes the state of a material instead of merely transporting it.
The effect is physical and local. A factory that needs a steady supply of Wet Sand should plan the source, the place where Sand meets Water, and the next processing step as one connected system. The official information does not give a universal ratio or production rate for a complete factory, so this page explains the interaction without turning it into a numeric optimization claim.
Sources and sustainable planning
The official Water page names surface lakes and underground water deposits as direct sources. Ice can melt into Water, and Steam reaching the sky can return as rain. Lumlings are also listed as a source. These options mean that “find Water” is not one single map instruction; the correct choice depends on what the current save exposes and which Research tools you have unlocked.
Water can eventually settle into pools. The official page documents an evaporation behavior for droplets that do not settle, but the exact timing is version-sensitive and should be rechecked before it is used as a production calculation. In practical terms, build a contained pool or a defined channel when you need reliable flow, and watch for droplets escaping at the edges.
Where Water can flow
Water flow is blocked by a Block, Conveyor Belt, Kinetic Press, and Flux Emanator. The source says it can pass through other building blocks, with Filters being an important example. Advanced Filters can affect liquids such as Water. That makes the building choice part of the recipe: a basic solid filter rule is not automatically a liquid filter rule.
Liquids can also slip through corners where two solid blocks meet if another block does not stop them. This is a small detail with large layout consequences. When a pool leaks, inspect corners and gaps before assuming that the source is empty or that the Pump is broken. Use a simple test cell to isolate the cause, then make the boundary intentional.
Water, Lava, and Steam
Water landing on Lava evaporates into Steam, while the Lava amount does not decrease. This is a documented interaction, not a promise of free infinite Water or a complete Steam loop. If the goal is to preserve a Water line, separate it from Lava; if the goal is to study the reaction, keep the test contained so the resulting Steam does not confuse the main production chain.
The safest Water habit is to plan for the next state. Water may be a source, an input to Wet Sand, an input to Wet Seed, or part of a heat-and-steam experiment. Name the purpose of each line and keep the source/date visible in your notes. That prevents a true interaction from being overextended into an unsupported factory ratio.