Imagine a microscopic robot. It has a body about the size of a human cell and 12 arms sticking out in all directions. A bucketfull of such robots might form a `robot crystal' by linking their arms up into a lattice structure. Now take a room, with people, furniture, and other objects in it — it's still mostly empty air. Fill the air completely full of robots. The robots are called Foglets and the substance they form is Utility Fog, which may have many useful medical applications.