In a small park, Hisako and Jabari are standing opposite each other with a large, opaque spherical sculpture between them. The sculpture has a unique property: it is perfectly smooth and reflects no light--it is a perfect "void" in visual perception. For this reason, neither Hisako nor Jabari can see each other. Initially, they are positioned so that a line drawn from Hisako to the sculpture and from the sculpture to Jabari forms a straight line. They both know that their friend Xolani is standing on top of a nearby hill with a vantage point that allows Xolani to see over the sculpture. Hisako and Jabari have decided they want to catch a glimpse of each other without moving laterally around the sculpture or changing their respective distances from it. Instead, they plan to gradually adjust their relative positions by moving directly away from the sculpture along the lines from their initial positions, while maintaining a line-of-sight connection through the top edge of the sculpture. As they move away from the sculpture, at what point will they be able to see each other over the top of the sphere, and what will Xolani observe during this process from her vantage point on the hill?
The sculpture's opaque quality and its spherical shape are both important to the scenario. As Hisako and Jabari move back, maintaining alignment with the sculpture, from the perspective of the ground level where they both are, their point of view of each other will initially remain blocked by the sculpture regardless of their distance from it, as the line tangent from their eyes to the top edge of the sphere will always be level. From their perspectives, they will only start to see each other when the tangent lines from their eyes just graze the topmost point of the sculpture, which will require both Hisako and Jabari to move back far enough that their lines of sight form a shallow 'V' over the top of the sphere.

Xolani, on the hill, has a different perspective. Since Xolani is elevated above the ground level, they are able to observe Jabari and Hisako from the beginning, as the hill's elevation provides a vantage point over the sculpture. As Hisako and Jabari move backward, aligning with the sphere's top edge from Xolani's perspective, they will gradually become aligned from her viewpoint with the top of the sphere. Eventually, when Hisako and Jabari reach the points where their lines of sight intersect just above the sculpture's top edge, they will be able to see each other. Xolani, due to their higher vantage point, will observe that Hisako and Jabari have stopped moving as soon as they come into each other's sight over the sculpture.

Throughout this process, to Xolani, Hisako and Jabari will appear farther apart than they initially were, and the height difference between their line of sight over the top of the sculpture and the sculpture itself will remain very minimal due to the straight path they are following away from the sculpture.