An impossible Penrose triangle, created using 3D printing. Various variations of this model are available in my shop. This is the smallest (and cheapest) version of this illusion.
As you may have guessed, given that it is available for sale, it's not really impossible. It only appears that way when viewed from a particular angle.
The shape is an interesting one, curvy and distorted, yet it can easily stand upright so you can showcase this intriguing illusion.