Yippee, my first animations in Mathematica!
Let a circle roll around a circle twice as big. The shape traced by a point on the outer circle is a cardioid. Now consider a third circle rolling around the second one as well (again half as big, and at the same speed); its trace is already less familiar. The more circles, the more fractal-ish the resulting curve will be. In the limit, the traced curve can be described with this parametric formula:
(Source of inspiration: http://www.mathrecreation.com/2013/12/brain-curve.html)