If one day, right before your eyes, an irregular geometric shape appeared out of thin air, like a living monster, madly twisting and deforming. One second it's a sharp pyramid, the next it becomes a complex octahedron. Finally, it twists and vanishes into thin air.Don't be quick to call this an illusion, because it very likely means a four-dimensional hypercube is passing through our three-dimensional world. What we see is merely the trace it leaves in the lower-dimensional world. It's not that we can't see ...