My old pair is quite sloppy (though not bad for a 10-year-old, I'd say. Good job, former me), so I decided to make a new and improved set--documented here on Instructables.
Now for the important part: what Napier's Bones are. They are a multiplication tool/math toy that allows you to multiply 0-9 with large numbers extremely quickly with only slabs of wood. As long as you add small numbers and read correctly, you will get the correct solution every time.