The magic number square has become my new hobby.
Meanwhile, you can buy many books, notebooks and notepads for light,
medium and heavy Sudokus.
Also for children a first-class brain and concentration training.
The rule is not difficult:
A Sodoku - box is a square that is divided into 3 x 3 smaller squares.
In some areas numbers are given. The puzzle over, the remaining numbers
from 1 to 9 to insert so that every row, every column and every 3 x 3 - Square, each digit from 1 to 9 only appears once.
There are also variations with the letters A - I.
Keeps the brain fit and is addictive!
Ideally for young and old.