An impossible Penrose triangle, created using 3D printing.
I have created multiple variants of this illusion. This is a normal sized version, consisting of cubes connected by a rod.
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.