An ordinary sudoku, plus thermometers: digits get bigger along every thermometer, starting from its round bulb. Rows, columns and boxes still hold 1 to 9 exactly once.
Mercury is a free daily thermo sudoku from Mind Loft. Each day gives you a 9×9 sudoku with thermometers drawn on the grid: along every thermometer the digits strictly increase from the round bulb to the tip. Rows, columns, and 3×3 boxes still hold 1 through 9 exactly once, and the thermometers plus a handful of printed digits carry the rest.
There is no arithmetic anywhere: the thermometers are pure ordering, which makes this the most visual sudoku on the slate. Every board has a single solution you can always reach by logic, with no guessing. Wrong entries are never flagged, so spotting your own slips is part of the puzzle, and a clean solve scores a perfect 10 with the daily leaderboard decided on the clock.
A new board drops every day at midnight Eastern; weekdays print fewer digits as the week goes on, and the Sunday Edition prints just eight and lets nine thermometers do the work. No app, no signup — play free in your browser. Want the rest of the family? Try Suds, the classic 9×9, Cages, the killer sudoku, and Polka, the kropki with no numbers at all.