Take the arresting image at the top of this page. Is it a clutch of translucent frog spawn, or perhaps green pearls of slime? In fact it is a close-up of microscopic algae spheres that appear trapped within a droplet of water. Incredibly, each green sphere is itself a colony made up of as many as 50,000 cells. The image—which was taken by chemical engineer Jan Rosenboom—won second place in the competition. And it is easy to see why.