[teknopaul]

I thought that one was the most interesting too. The convex hull appears circular but it is not, in one case you have to join the dots between the center point of triangles, none of the points on the triangles are on a circle, or maybe all of them are if we discard the hull abstraction, in which case there are three circles.

Imagine trying to write code that identifies that. However it's one of the most obvious to me.