Sculpture is three-dimensional art Sculpture is three-dimensional art. You don't just look at a flat surface. Sculpture has a front, back, sides, top and bottom. You can move around the artwork. In the 1930s, American artist Alexander Calder made sculpture move around you. He created a kind of art called mobiles, sculptures of wire and metal shapes usually hung from the ceiling. The parts were hinged together so that each part can move on its own. The sculptures can be very small or very large. Some are moved by motors though most are pushed slowly by the air currents in a room. One thing all mobiles have in common is that they are all carefully balanced. The shapes hung from the wires must be just the right weight and stick out just far enough to keep the sculpture arms in the right position. Calder used curved shapes in his mobiles, like the "life" shapes used in surrealism. Where most sculptures have the three dimensions of height, width, and depth, Calder's mobiles add a fourth dimension: time. It takes time to enjoy these sculptures. You need to take time to watch how the pieces move.

Calder succeeded in integrating natural movement into sculpture by assembling elements that balance themselves naturally by weight, surface area, and length of wire "arm." The basic equilibrium he struck guarantees compositional harmony among the parts, no matter their relative positions at any given moment. Though many other artists have since created works based on his principles, even now, decades later, Calder is still the undisputed master of this form of sculpture.