Sudoku
It is like a crossword puzzle using numbers instead of words.
The puzzle is a box of 81 squares. Some squares are filled in with numbers. The rest are there to be filled in by the puzzler.
Fill in the blank squares allowing the numbers 1-9 to appear only once in every, row, column and 3x3 box.
Fill in the grid so that every row, every column, and every 3x3 box contains the digits 1 through 9. It really is that simple.