Tesseract

Physics is a very interesting subject. Take, for example, the tesseract.

A two-dimensional being (let's call him "Mr. 2D") cannot imagine/envision a cube. If you unfolded that cube, you'd have a cross made of six squares. Mr. 2D could "see" it. But once you begin to re-fold it, the sides would seem to disappear, to him. He'd only see a two-dimensional slice of it. As it spins, he'd see a square that seems to change shape. Mr. 2D lacks the ability to perceive the third dimension.

A tesseract is a "cross" made of eight cubes. When folded, these cubes form a "hypercube". When we look at a spinning hypercube, we'd see a strange shape changes, but it wouldn't make sense in a three-dimensional world. We lack the ability to perceive the fourth dimension.