---
title: How to Play Thermometers Puzzles: Rules and a Worked Example
description: The rules of the Thermometers logic puzzle in plain words, with a 4x4 example solved step by step. Then play 660 more in Mercuro, free on iPhone and iPad.
url: https://www.arbazsiddiqui.me/app/mercuro/how-to-play-thermometers/
---

# How to play Thermometers puzzles

Three rules, one small grid cracked move by move, and the habits that make a 12x12 feel like a 5x5.

App Store: https://apps.apple.com/app/id6762402072

## Three rules

1. The number beside each row or column says how many of its cells hold mercury.
2. Mercury starts in the bulb, the round end, and rises toward the other end without gaps.
3. A thermometer can be empty, full or filled partway.

A well-made puzzle has exactly one answer, and logic always reaches it. Guessing never comes into it.

## Your first grid

Six thermometers on a 4x4 grid. Row numbers run down the left, column numbers along the top, and both count from the top-left corner. Solve it before you read the steps.

## The solve, move by move

1. Row 3 has a 0, so all four of its cells are empty.
2. Column 2 needs 3 filled cells. Its row 3 cell is empty, which leaves exactly three cells (rows 1, 2 and 4), so all three fill.
3. Row 1, column 2 is filled, and it sits in the top thermometer. Mercury can't skip a cell, so the bulb to its left fills too.
4. An empty cell cuts off everything beyond it. The left thermometer starts in row 2 and is empty in row 3, so its row 4 cell is empty. The right thermometer is empty in row 3, so its row 4 cell is empty as well.
5. Row 4 needs 2. Column 2 is already filled and columns 1 and 4 are empty, so column 3 fills.
6. Column 1 needs 2, and rows 3 and 4 are empty, so rows 1 and 2 fill.
7. Column 3 needs 1, and row 4 already has it, so rows 1 and 2 of column 3 are empty.
8. Column 4 needs 2, and rows 3 and 4 are empty, so rows 1 and 2 fill. Rows 1 and 2 now have 3 each, which matches their numbers. Solved.

## Habits of fast solvers

- Hunt the 0s and the full lines first. Each one settles a whole row or column in a single move.
- A filled cell drags every cell between it and the bulb up with it.
- An empty cell cuts off everything beyond it, away from the bulb.
- Count what is left. A line that needs two more with two open cells fills both.

## Questions

**Which end is the bulb?** The round end. Mercury always starts there, whichever way the thermometer points.

**Is there always exactly one solution?** In Mercuro, yes. Every board is checked by a solver to have exactly one answer.

**How is it different from a nonogram?** A nonogram clue describes runs of filled cells. A Thermometers clue is a single count, and the thermometer shapes decide which cells can fill first.

**Where can I play more?** Mercuro has 660 Thermometers puzzles on iPhone and iPad, 330 of them free, plus three new ones every day.