Colloidal particles, used in a range of technical applications including foods, inks, paints, and cosmetics, can self-assemble into a remarkable variety of densely-packed crystalline structures. For decades, though, researchers have been trying to coax colloidal spheres to arranging themselves into much more sparsely populated lattices in order to unleash potentially valuable optical properties. These structures, called photonic crystals, could increase the efficiency of lasers, further miniaturize optical components, and vastly increase engineers’ ability to control the flow of light.