God told Adam and Eve, "Be fruitful and multiply, and replenish the earth." A robot that received the same command would feel either flummoxed or frustrated. Why? Because self-replication has proven elusive. It's one thing to build a robot -- it's another thing entirely to build a robot that can make copies of itself or regenerate lost or damaged components.
Interestingly, robots may not look to humans as reproductive role models. Perhaps you've noticed that we don't actually divide into two identical pieces. Simple animals, however, do this all of the time. Relatives of jellyfish known as hydra practice a form of asexual reproduction known as budding: A small sac balloons outward from the body of the parent and then breaks off to become a new, genetically identical individual.
Scientists are working on robots that can carry out this basic cloning procedure. Many of these robots are built from repeating elements, usually cubes, that contain identical machinery and the program for self-replication. The cubes have magnets on their surfaces so they can attach to and detach from other cubes nearby. And each cube is divided into two pieces along a diagonal so each half can swivel independently. A complete robot, then, consists of several cubes arranged in a specific configuration. As long as a supply of cubes is available, a single robot can bend over, remove cubes from its "body" to seed a new machine and then pick up building blocks from the stash until two fully formed robots are standing side by side.