If you want to understand how crystals begin to form on a flat surface, just think about adding oranges to a cardboard box: each packed side by side with its neighbors. Change the speed of when individual crystal components join and you can get unusual shapes, such as snowflakes. But try to grow a crystal on a curved surface, and this regular arrangement breaks apart. Now, researchers have managed to track this effect in exquisite detail. They suspended tiny polystyrene particles in spherical droplets of water and watched through a microscope as the particles moved about (see video). The particles’ chemical makeup initially attracts them to the inside edge of the droplets where the water meets the surrounding air, then to their neighbors. As successive particles pack together, crystallites begin to coat the inside of the droplet. But the droplet’s curvature increases a property known as the elastic tension, forcing the crystallites to grow into ribbonlike structures that mesh together in patches as more continue to assemble on the inner surface of the droplets. By understanding this process, reported online today in Science, researchers say they hope to better determine how curved nanoscale objects, such as viruses, put themselves together.