Desktop users may use a keyboard and mouse to enter numbers. Mobile users may use the keypad. Or if you are on the desktop and like clicking a lot, you may also use the keypad.
The rules are pretty simple, each row, column and 3x3 square must contain the numbers 1-9 with no duplicates.
Medium and Hard levels may take a few seconds to load.
Internet Explorer may complain about the script taking too long and ask if you want to stop it. You may need to click No several times. I may fix this in a future version (but I doubt it).