Visually stunning math concepts which are easy to explain Since I'm not that good at (as I like to call it) 'die-hard-mathematics', I've always liked concepts like the golden ratio or the dragon curve, which are easy to understand and explain but are mathematically beautiful at the same time.

Do you know of any other concepts like these?
I think if you look at this animation and think about it long enough, you'll understand:


Why circles and right-angle triangles and angles are all related.
Why sine is "opposite over hypotenuse" and so on.
Why cosine is simply sine but offset by $\frac{\pi}{2}$ radians.
$\Bbb RP^2\sharp\Bbb RP^2\simeq \text{Klein Bottle}$