 This is a mathematical snack from the Association of Teachers of Mathematics. You need to draw yourself a square, as near to a square as you can draw, and choose four numbers, one for each of the corners of the square. So seven, maybe, five, three, two. Now we're going to find the difference between each of the two numbers on the side, so the difference between is seven and two. We're going to write in the middle here is five, and the difference between seven and five is two. So we write it there, and we find the difference between five and three is two, and the difference between two and three is one. Now I have a new square made up of my four new numbers, and we're going to do the same again. The difference between five and two is three. Between two and two, nothing. Between two and one, one. Between five and one, it's four. And we repeat that with the four new numbers. The difference between three and four is one. The difference between three and zero is three. The difference between zero and one is one. And the difference between four and one is three. Keep going. New square, the new difference. Difference between three and one is two. Between three and one is two. Between three and one is two. Between three and one is two. And now you might realize why this snack is called naughty numbers. So my challenge to you is to find out what happens if you change the numbers on the original corners of the square. Do you still end up back at nought? Enjoy.