What would it look like if a four-dimensional Stranger—a “hypersphere”—were to pass through our three-dimensional Spaceland? First we would see nothing; then a point-like ball would appear, expand into a sphere, then dwindle to a point and vanish. The spheres of varying sizes that we saw would be 3-D cross sections of the hypersphere, just as the circles that Mr. Square saw were 2-D cross sections of the sphere.
