Water sort.
Keep a little space.

What moves in one turn

Each tube holds four units. Select a nonempty source, then choose a destination that is empty or has the same color on top. One move pours as much of the source’s continuous top-color segment as will fit. You cannot stop a pour early or pour through a different color underneath.

For example, a source ending with two B units can pour only one into a destination already holding three B units. The source keeps the other B. A tube showing A on top cannot accept B, even if it has room. Filling four mixed units is not a completed tube.

Finish with every nonempty tube full of a single color. Empty tubes may remain. Every starting layout here has a route verified to that goal. A sequence of poor pours can still reach a dead end; Undo and Restart are part of learning the board.

Colors have letters, too

A through E label the colors consistently within a board. You do not need to distinguish colors by hue alone. A tube’s accessible label reads its contents from bottom to top and includes how much of its capacity is filled.

Touch and keyboard

Tap a tube containing water, then tap its destination. Tap the selected source again to cancel. An invalid pour leaves the water, move count and clock unchanged. To use a keyboard, focus the tube board: left and right choose a tube, up and down jump four positions, and Enter or Space selects or pours. Escape clears a selection. The outlined tube is your keyboard aim.

Hint selects a source and marks a destination from a fully verified route. Press Enter or tap the suggested destination to make that pour. It does not move water or start time by itself. After a detour, the solver searches again; if it cannot verify a complete route within its search budget, it says so and recommends undo or restart instead of inventing a move.

The first legal pour starts time. Pause hides the tubes and freezes the clock; leaving the tab pauses an active attempt. Reloading in the same tab restores it paused. Undo reverses the whole pour and counts as a move. Restart returns to the same layout and keeps any Assisted status. New board rotates through three original layouts for the chosen difficulty and starts a fresh Solo attempt.

Reserve space for a split, not just a match

Two spare tubes are useful because one can hold a temporary color while the other gives you somewhere to merge. There is no rule that you must keep exactly two tubes empty at every moment. What matters is whether you can free enough capacity to expose the color you need next.

Before consuming the last empty tube, name the pour that will empty a different one. If that next pour does not fit, the visible match may leave you with no useful buffer. A partial monochrome tube is still storage in use; only a full monochrome tube is finished.

Follow the bottom colors

Look at which tubes hold a color beneath other units. Clearing the top of one of those tubes can unlock several later matches. Combining top segments is useful when it exposes that buried color or frees a tube, not merely because the two top letters match.