You don’t need rain, or even a sprinkler, to see rainbows. In this exhibit, visitors make rainbows by shining a light at a layer of the glass beads that make highway signs and street markings highly reflective. Rainbows in sunlight are always circular arcs because the source is effectively at infinity, so all incoming rays are parallel. Circular rainbows can be seen at our exhibit too, as special cases of a whole family of curves determined by the relative positions of the eye, the light source, and the plane of the glass beads. The arcs are circular when the line through the eye and the source is perpendicular to the plane of the beads.