Place the numbers 1-9 in the grid so that every row, every column, and each of the nine 3x3 square contains only one occurence of all the numbers from one to nine.
You should be able to solve any puzzle here without writing anything down, and they all have only one solution. If you feel you can't go, you can click the 'give me a hint' button, and any errors you have made will be highlighted in red, or if you've made no mistakes, a green square will show where you can go next. If you try to place a number where it already exists on that row, column, or small square, both occurences will be highlighted in red.