Learning how to play Grids of Thermometers is about mastering two simple rules of logic. The goal is to fill each thermometer shape on the grid with "mercury" so the total number of filled cells in every row and column exactly matches the numbers written on the outside. Every puzzle is solved with pure deduction—no guessing required.
This guide breaks down the fundamental rules, a step-by-step process for your first solve, and the core strategies you'll need to crack any puzzle, from a simple 5x5 to a sprawling grid.
What Are the Core Rules?
Every Grids of Thermometers puzzle is governed by a strict, physics-based logic that makes it solvable. Once you internalize these three concepts, you'll have the foundation for every other technique.
The Anatomy of the Puzzle
A puzzle consists of three main parts:
- The Grid: A square or rectangular area where the puzzle takes place.
- The Thermometers: Shapes of varying lengths placed on the grid. Each has a round bulb and a stem. They can be oriented vertically, horizontally, or even diagonally in more advanced puzzles.
- The Clues: Numbers placed at the end of each row and the top of each column. These numbers tell you the exact count of filled cells required for that specific line.
The Golden Rule of Filling
This is the most important rule: mercury always fills continuously from the bulb outwards. You can never have a filled cell floating in the middle of a thermometer's stem with an empty cell between it and the bulb. If you decide a cell must be filled, you must also fill every cell between it and its bulb. A thermometer can be completely full, completely empty, or partially filled, but never with gaps.
The Cascade Effect of Empties
This rule is the inverse of the filling rule. If you determine that a cell must be empty, then every cell further away from the bulb in that same thermometer must also be empty. For example, if you mark the third cell of a five-cell thermometer as empty, you instantly know that the fourth and fifth cells must be empty as well. This is because the mercury could never reach them without passing through the now-empty third cell.
Your First Moves: A Step-by-Step Walkthrough
Let's apply the rules to a new puzzle. The process is iterative, with each deduction revealing new information that makes the next step possible. Solving is less about a single flash of insight and more about methodically applying logic.
Step 1: Find the Certainties (0s and Full Lines)
Always start by scanning the clue numbers for the easiest deductions.
- Zero Clues: If a row or column has a '0' next to it, you can immediately mark every single cell in that line with an 'X' to signify it's empty. This is the most powerful starting move you can get.
- Full Line Clues: If a row has a clue of '6' and there are only six cells in that row, you can fill all of them. These are less common but provide a huge head start.
Step 2: Apply the Filling and Emptying Rules
Now, look at the consequences of your first moves. If marking a row 'empty' in Step 1 placed an 'X' in the middle of a thermometer, you now know all the cells above it (away from the bulb) must also be empty. Mark them immediately. Conversely, if you filled a cell, make sure every cell between it and the bulb is also filled. This is the "bulb cascade."
Grids of Thermometers in-game screenshot
Step 3: Cross-Reference and Reduce Options
This is where the real puzzle begins. Look at a row with a clue of, say, '3'. If you've already filled one cell in that row due to a column clue, you now know you only need two more filled cells in that row. This new information dramatically reduces the possibilities. Similarly, once you have found all three filled cells for that row, you can mark every other available cell in that row as empty.
Step 4: Iterate Until Solved
Solving a Thermometers puzzle is a cycle. Mark a cell, see what that implies for its thermometer, then see what it implies for its row and column, and then repeat the process with the new information you've gained. Continue this loop of deduction until every cell is either filled with mercury or marked as empty, and all row and column clues are satisfied.
Essential Beginner Strategies
Once you've mastered the basics, you can start thinking more tactically. These strategies help you find points of leverage in a grid that seems stuck.
Focus on High and Low Numbers
The most informative clues are the extremes. A '0' or a '1' gives you a lot of certainty. A high number, like a '5' in a 6-cell row, is also very powerful. It tells you that at least four of the cells must be filled, regardless of which one is left empty. This can often be enough to partially fill several thermometers and give you the information you need to solve an intersecting line.
Use the "Must-Be-Filled" Logic
Consider a row with a clue of '3'. Now, look at the thermometers in that row. Imagine two of the cells belong to thermometers whose bulbs are in other rows, and you've already proven those bulbs must be empty. This means those two cells can't be filled. If there are only three other cells remaining in the row, they must be the filled ones. You can fill them with confidence.
Grids of Thermometers in-game screenshot
Master the "Must-Be-Empty" Logic
This is the opposite and equally powerful. Imagine a vertical thermometer that is 4 cells tall. Its bulb is in a row with a clue of '1'. Because mercury has to fill from the bulb, you instantly know that the top 3 cells of that thermometer must be empty. Even if you don't know which cell in the row is the filled one yet, you know it can't be one that requires filling four cells in a single thermometer. This technique, sometimes called "overflowing," is crucial for harder puzzles.
What If You Get Stuck?
It's normal to hit a wall where no obvious moves seem available. When this happens, resist the urge to guess. The solution is always in the logic.
Re-scan All Clues
This is the most common fix. Go back and check every single row and column clue one by one. It's incredibly easy to complete a row (e.g., find all 3 filled cells for a '3' clue) and forget to mark the remaining cells in that row as empty. This one forgotten 'X' is often the key to unlocking the next step.
Hunt for Critical Intersections
Look for a single cell that is constrained by powerful clues in both its row and its column. For example, a cell in a row with a clue of '1' and a column with a clue of '1' is a point of high tension. Deducing the state of this one cell often has a cascading effect, providing definitive information about both its row and column simultaneously.
Grids of Thermometers in-game screenshot
The Assumption Technique (For Experts)
On exceptionally difficult puzzles, you might be forced into a technique known as bifurcation, or a forced move. Choose a single cell that can only be one of two states (filled or empty). Provisionally assume it's filled. Follow the chain of logic as far as you can. If you arrive at a contradiction (e.g., a row needs 4 fills but you can only make 3), your initial assumption was wrong. You can now confidently mark that cell as empty. This should be a last resort, as a mistake can unravel your entire solution, but it's a valid tool for advanced puzzles.
Frequently Asked Questions
Can a thermometer be completely empty? Yes. If the logic dictates, a thermometer can have zero filled cells.
Can a thermometer be completely full? Absolutely, as long as filling it completely does not violate any of the row or column clue counts.
Do the numbers refer to filled thermometers or filled cells? The clues refer to the total number of individual filled cells (or segments) in a row or column, not the number of thermometers.
Is there always only one unique solution? Yes, a properly constructed Grids of Thermometers puzzle will always have exactly one solution that can be reached through logic alone.
The Final Temperature Check
Grids of Thermometers might look complex, but it's a game of pure, satisfying deduction built on a handful of simple rules. The joy comes from the cascade of logic, where placing a single 'X' can suddenly solve an entire quadrant of the grid. Start with the certainties, always respect the bulb rule, and work your way through the layers of inference. With practice, you'll learn to see the patterns and dependencies, turning a confusing grid into a finished masterpiece of logic.