Sierpinski's Triangle.

But the numbers in the next diagonal are called the triangular numbers because if you take that many dots,you can stack them into equilateral triangles.The next diagonal has the tetrahedral numbers because similarly, you can stack that many spheres into tetrahedra.Or how about this:shade in all of the odd numbers.It doesn't look like much when the triangle's small,but if you add thousands of rows,you get a fractal known as Sierpinski's Triangle.