Mercury (Mind Loft)

Mercury is the thermo sudoku on Mind Loft, a free site of daily browser games. The grid is an ordinary 9x9 sudoku: each row, column and 3x3 box holds the digits 1 to 9 once. On top of that, grey thermometers snake across the cells, each with a round bulb at one end. Along every thermometer the digits must strictly increase from the bulb outward.

There are no sums to work out. The thermometers only set an order, which makes long ones very strong: a thermometer of six cells leaves little room for the digits that can sit in it. Together with a few printed digits, they fix a single solution that can be reached by logic alone, without guessing.

The game does not mark wrong entries, so catching your own mistakes is part of the solve. A clean solve scores 10, and the day's leaderboard is ordered by time. The board has the usual helpers: notes, auto-move, undo, erase and a hint button, plus a count of how many of each digit you still have to place.

A new board goes up at midnight US Eastern Time. Through the week fewer digits are printed, and the Sunday Edition gives just eight with nine thermometers. Mind Loft has a family of sudoku dailies beside it, including Suds (classic), Cages (killer) and Polka (kropki).