The artist Nagai Hideyuki plays with our sense of perception by drawing figures on two flat pieces of paper that, when they are held at right angles and viewed from a certain direction, give the illusion of being 3-D constructions.
Here is one example and you should go to his website to see more.
What impresses me about artists like Hideyuki is how they manage to conceive of such ideas and then actually bring them to fruition. The ‘simple’ way would be to construct a 3-D model and then sketch what you see. But I suspect that Hideyuki is not doing something that pedestrian but instead visualizes the whole thing and then draws it. It is really quite ingenious.