Self-replicating robots are no longer the stuff of science fiction. Scientists at Cornell University in Ithaca, N.Y., have created small robots that can build copies of themselves. Each robot consists of several 4-inch cubes that have identical machinery, a computer program for replication, and electromagnets to attach and detach to one another. The robots can bend and pick up and stack the cubes. The machines duplicate themselves by bending over and putting their top cube on the table. Then they bend again, pick up another cube, put it on top of the first and repeat the entire process. As the new robot begins to take shape it helps to build itself.