In fact: every 3D work we see on our monitor simply creates the illusion of being 3D.

We've been talking about 2D apps creating 3D illusions too, but, when drawing in 2D, things can go wrong. The way we create the illusion of 3D is called "perspective", and the mathematical background for this was invented during the renaissance. (15th, 16th century)

In the following drawings you can see how using the ellips tool can create anomalies in perspective, and what can be done about it. By the way: mathematical perspective itself is also very limited.