Normal sudoku rules apply. Entering correct digits may clear fog somewhere in the grid. No guessing is necessary. In this puzzle, coloured lines are 'edge thermos'... these run along cell edges. Any cell edge covered by a thermo contains a thermo spot. The value of a thermo spot equals the sum of the digits in the two cells it sits between. Values of spots along a thermo strictly increase moving away from the bulb (the large spot at one end.) Each thermo is a different colour, and they don't overlap or intersect each other.