Their robots are made up of a series of modular cubes -- called "molecubes" -- each containing identicalmachinery and the complete computer program for replication. The cubes have electromagnets on theirfaces that allow them to selectively attach to and detach from one another, and a complete robot consists ofseveral cubes linked together. Each cube is divided in half along a long diagonal, which allows a robotcomposed of many cubes to bend, reconfigure and manipulate other cubes. For example, a tower of cubescan bend itself over at a right angle to pick up another cube. To begin replication, the stack of cubes bends over and sets its top cube on the table. Then it bends to oneside or another to pick up a new cube and deposit it on top of the first. By repeating the process, one robotmade up of a stack of cubes can create another just like itself. Since one robot cannot reach across anotherrobot of the same height, the robot being built assists in completing its own construction. Although these experimental robots work only in the limited laboratory environment, Lipson suggests thatthe idea of making self-replicating robots out of self-contained modules could be used to build workingrobots that could self-repair by replacing defective modules. For example, robots sent to explore Marscould carry a supply of spare modules to use for repairing or rebuilding as needed, allowing for moreflexible, versatile and robust missions. Self-replication and repair also could be crucial for robots workingin environments where a human with a screwdriver couldn't survive.