Golden Rectangle
The Golden Rectangle is a rectangle where the ratio of length : width equals Phi.
You'll notice that the rectangle above contains smaller rectangles within. When you take a square out of the Golden Rectangle with side length equal to the width, or smaller side of the rectangle, the piece that remains is also a Golden Rectangle.
Repeating this creates the colorful rectangle above, which then allows you to create the Golden Spiral, one of the most beautiful shapes found everywhere in nature.