Here is a $100$ square with some of the numbers shaded:

Look at the green square which contains the numbers $2, 3, 12$ and $13$.
What is the sum of the numbers that are diagonally opposite each other?
Do you notice anything?
Look at the pink square.
Does the same thing happen?
Look at the squares shaded red. They form the corners of a large $3$ by $3$ square.