8 common thermometer puzzle mistakes

None of these mistakes come from not knowing an advanced technique. They are slips in applying the basic rules, and they are easy to fix once you know what to look for.

1. Leaving a gap in the mercury

Mercury must be one unbroken column from the bulb. A filled cell with an empty cell between it and the bulb is never valid, even if every clue is matched.

Fix: Turn on Highlight rule breaks in Settings, and use auto-flood: tapping a cell fills everything back to the bulb for you.

2. Filling from the wrong end

The bulb is the rounded end, and it is not always on the left or at the top. On a thermometer pointing left, the mercury starts on the right.

Fix: Before counting a line, glance at where each bulb is. On curved puzzles, trace the tube from the bulb.

3. Counting a crossing thermometer as more than one cell

A straight thermometer that crosses a row puts exactly one cell into that row. When estimating how much a row can hold, it counts as 0 or 1, however long it is.

Fix: For each line, separate the thermometers that lie along it from those that cross it. Only curved tubes can visit a line more than once.

4. Using the overlap rule on the wrong thermometers

The overlap rule from first moves (fill L + c − n cells) only applies to a thermometer that lies along the line. Applying it to a crossing thermometer gives nonsense.

Fix: When in doubt, fall back on the general rule from the squeeze and the ceiling: the share of one thermometer is clue minus what the rest can supply.

5. Deciding a cell but not spreading it

Every new mercury cell fills back to the bulb, and every new air cell empties forward to the tip, often into other rows and columns. Forgetting to spread means missing the next deduction.

Fix: After each move, ask "which other lines did that touch?" and check those next.

6. Not marking air

Marking air is optional, but solvers who skip it lose track of which cells are still open. Counting open cells is the basis of half the deductions in the puzzle.

Fix: Mark air as soon as you know it. Right-click, long-press or press X in our game. Clicking a satisfied clue marks the rest of its line in one go.

7. Reading the wrong clue

On large grids it is easy to read the clue for row 9 while looking at row 8. The deduction looks valid but places mercury in the wrong line.

Fix: Hover over a cell in the game: its row and column are highlighted. On paper, use a ruler or the edge of a sheet.

8. Guessing too early

Every puzzle on this site can be solved without guessing. A guess made because you are stuck usually means a squeeze or ceiling was missed somewhere, and a wrong guess can cost far more time than the search for the real deduction.

Fix: Re-check the lines with the least slack first. If you do make a supposition, remember your move count so you can undo back to it cleanly.

How to catch mistakes early

• Check as you go: the game's Check button lists wrong cells without revealing the answer.
• Watch the clues: a clue turns red the moment its line is over-full or can no longer reach its target.
• Use the totals: the sum of the row clues equals the sum of the column clues. That is the total amount of mercury in the solution.

Questions