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Thermo Sudoku is a variant where thermometer shapes are drawn on the grid. Each thermometer has a bulb (rounded end) and extends through several cells toward a tip. The digits must strictly increase from the bulb to the tip — so every cell along a thermometer must hold a larger digit than the one before it.

The bulb is the starting point and must hold the smallest digit on that thermometer. Each subsequent cell must be strictly larger. For example, a 3-cell thermometer starting at the bulb means the bulb holds at most 7 (since two larger digits must follow). A 5-cell thermometer means the bulb holds at most 5.

No. Because digits must strictly increase, no digit can appear twice on the same thermometer. A 4-cell thermometer uses four different increasing digits. Combined with standard Sudoku uniqueness rules, thermometers create very tight constraints on every cell they pass through.

The core rule is simple — digits increase from bulb to tip. Beginners can start solving after understanding just that one constraint. What makes higher difficulties challenging is tracking minimum and maximum values for each thermometer cell simultaneously across many overlapping thermometers.

Look for long thermometers. A 7-cell thermometer can only start at 1 and end at 7, 8, or 9 — drastically limiting all seven cells. Also check for thermometers where the bulb or tip overlaps with a constrained row, column, or box, as this often pins the starting digit immediately.

Easy puzzles take 8–15 minutes. Medium puzzles typically run 15–30 minutes. Hard puzzles average 35–60 minutes and expert puzzles can exceed 90 minutes. The difficulty scales sharply as thermometer lengths decrease and the number of simultaneously active constraints increases.