Put in perspective
Used for poetic effect in Alex Garland's novel, a tesseract is a "hypercube," a four-dimensional shape whose "shadow" would be a three-dimensional cube. Imagine a cube surrounded by cubes. This Escher-like diagram shows four of those surrounding cubes distorted to represent their touching but perpendicular sides. Put it all together - in the fourth dimension, please - and you've got a tesseract.