Make the numbers in the row or in the column to total sum number at the end, using numbers 0-9. Start with simple 3 x 3 board with three of the numbers filled in. Once you have mastered the 3 x 3 you can go onto the 4 x 4, which has eight numbers unlocked. 5 x 5 board coming later this week, stay tuned! Plans to release an Apple Watch version in the works as well.