Fill in the blanks so that every number from 1-9 appears in each of the nine columns, in each of the nine horizontal rows and in each of the nine 3x3 boxes that make up the grid. Numbers can only be used once in each row, column or box.

Select a blank square by left-clicking on it. Enter a number between 1 and 9. If it turns red it is a duplicate; if it turns blue it could be correctly placed.

If you're not sure of an answer you can "pencil in" numbers by holding down the space bar, left-clicking the mouse and then typing in the required number from 1 to 9.

Solve the puzzle by filling all the squares in the grid with numbers. Every horizontal row, vertical column and 3x3 box should contain the numbers 1 thru 9 with no duplicates.