This puzzle is rated 1 star (medium).
I thought it best to include this puzzle now, rather than later, because it is so famous it will be expected to be on here somewhere. If you have seen this before it will be easy, but otherwise I think a medium rating is most appropraite.
Using the grid described above, place the numbers 1, 2, 3, 4, 5, 6, 7, 8 and 9 in the grid so that all the horizontal, vertical and diagonal lines add up to exactly 15.
Check the solutions page for the correct answer.