Motion is the central principle behind this public artwork, planned especially for Willis Tower. The dynamic pattern on the wall is activated by the motion of people walking, driving, or biking past; by the motion of the earth in relation to the sun as light moves across it; and by changes in the season and weather. Viewing the work from various positions and at various times of day produces a dramatically different experience. The artwork covers the wall with a pattern of metal tiles based on Penrose tiling. Discovered by mathematician and physicist Sir Roger Penrose in the 1970s, this approach produces a system of non-periodic tiling that is based on five-fold symmetry. The result feels both regular and random, hovering just beyond our ability to quickly comprehend it. Each tile is curved, a fragment of the inner surface of a sphere, and the main tones used in the work – blue, deep green, and white – are redolent of the surfaces of nearby Lake Michigan and the Chicago River. The concave shapes and colors of the tiles produce a dynamic effect when visitors walk around it. Seen from certain angles, the pattern reveals a vortex that seems to twist and accelerate in response to viewers’ movements. The enameled steel gently catches the light of the sun; the concave surfaces collect shadows that shift as the day progresses. At night the work is lit from behind so that flashes of light escape through the interstices between the tiles. As viewers move, the pattern of light appears to move with them, revealing the underlying geometry of the work and creating a captivating effect that activates the street around the building at night, attracting visitors at all hours. The used product was custom-made for Studio Olafur Eliasson.