Everybody knows a computer is a machine made of metal and plastic, with microchip cores turning streams of electrons into digital reality.
A century from now, though, computers could look quite different. They might be made from neurons and chemical baths, from bacterial colonies and pure light, unrecognizable to our old-fashioned 21st century eyes.
What is a complex system? In general terms, a complex system consists of a number of interacting 'members', 'elements' or 'agents', which have the potential to generate qualitatively new collective behaviour.