How to play thermometer puzzles
Everything you need to start solving: the three rules, what valid and invalid fills look like, the game controls and a complete solved example.
The goal
Fill some of the thermometers with mercury so that every row and column contains exactly as many mercury cells as its clue says. Mercury always fills a thermometer from the bulb end and never leaves a gap.
A thermometer puzzle is a square grid divided into thermometers. Each thermometer has a round bulb at one end and a flat tip at the other, and covers between one and eight cells. Every cell belongs to exactly one thermometer. Numbers along the top give a target for each column, and numbers on the left give a target for each row.
The rules
- Mercury starts at the bulb. If a thermometer contains any mercury at all, its bulb is filled.
- Mercury is continuous. The filled part runs from the bulb toward the tip with no empty cells in between. A thermometer can be empty, partly filled or full.
- Clues count mercury cells. A clue of 3 next to a row means exactly three cells in that row are filled. It doesn't matter which thermometers they come from.
Valid vs invalid fills
Here is one five-cell thermometer, with its bulb on the left, filled four different ways:
Game controls
| Action | Mouse | Touch | Keyboard |
|---|---|---|---|
| Fill mercury up to a cell | Left-click | Tap | Space / Enter / M |
| Mark air (empty) from a cell to the tip | Right-click or Shift-click | Long-press, or Air mode | X |
| Fill several cells in one motion | Click and drag | Tap and drag | — |
| Clear a cell | Click again | Tap again | Backspace |
| Finish a row or column | Click its clue | Tap its clue | — |
| Undo / redo | Undo and Redo buttons | Ctrl+Z / Ctrl+Y | |
| Smart hint | Hint button | H | |
Auto-flood (on by default) fills every cell from the bulb up to the one you click, and marking air empties everything from that cell to the tip. Clicking a clue whose line is already satisfied marks all of its remaining cells as air. If the remaining cells must all be mercury, clicking the clue fills them.
Worked example: solving a 4×4 puzzle
Let's solve this small puzzle step by step. There are six thermometers. The row clues on the left read 4, 1, 0, 1 from top to bottom, and the column clues on top read 1, 3, 1, 1.
Step 1: the free lines
Row 1 has a clue of 4 in a 4-wide grid, so every cell in it is mercury. Row 3 has a clue of 0, so every cell in it is air. Since air also blocks everything beyond it toward the tip, the two thermometers that run down through row 3 (in columns 1 and 4) are also empty in row 4. So is the short thermometer whose bulb sits in row 3, column 3.
Step 2: finish the satisfied columns
Columns 1, 3 and 4 each have a clue of 1, and each already has its single mercury cell in row 1. Every other open cell in those columns must be air.
Step 3: row 2
Row 2 needs 1 mercury cell, and only one open cell is left in it: column 2, which is the bulb of a short thermometer. It must be mercury.
Step 4: row 4
Row 4 needs 1 mercury cell and has one open cell left: column 2, which is the bulb of an upward thermometer. Fill it. Column 2 now has 3 mercury cells, matching its clue, and the thermometer's tip in row 3 stays empty. Solved!
Larger puzzles use exactly the same reasoning, just with longer chains. Try a 6×6 easy puzzle next, then read about advanced strategies.
Beginner tips
- Mark air generously. Crossing out cells you know are empty makes the remaining open cells easy to count.
- Always think "from the bulb". Filling a cell fills everything behind it; emptying a cell empties everything ahead of it.
- Track the remaining count. In Settings, turn on Show remaining counts so each clue counts down as you fill.
- Use hints to learn. A hint fills a cell you could have deduced and names its row and column. Look at those lines to see why.
Common questions
Can a thermometer be completely empty?
Yes. A thermometer can hold any amount of mercury from zero up to its full length.
Can a thermometer be completely full?
Yes. A full thermometer is valid as long as its cells are allowed by the row and column clues.
Do I have to mark empty cells?
No. You only need to place the mercury correctly. Marking empty cells as air (×) is optional, but it helps you keep track of your deductions.
Ready to try?
Start with a gentle 6×6 puzzle.