Cubes with neodymium magnets in the walls, so they stick to each other. Any two sides of any two cubes will attach, with no polarity problems, since the spherical magnets spin inside their chambers to line up with each other. http://www.youtube.com/watch?v=QwNefJWW3nc See the instructions for how to get the magnets in there. I put neodymium magnets inside the walls of these cubes and they make a cool stress relief gizmo for adults. The magnets are sold as Zen Magnets and several other names. I own Zen Magnets, they're the cheapest. They are not toys, a company selling them as toys called Buckyballs was sued. The idea here is to drop magnets into the internal cavities of the cubes before they are done printing, so the magnets are trapped inside the cubes. That's why the two sets of cubes are far apart, so you can pause the printer while it's hovering over one set, and drop the magnets into the other cubes. Careful here! Be sure the cavities are just about finished printing before you drop in the magnets, or else the extruder will hit the magents. You have to watch the print carefully and be ready to pause it, which is a pain I admit but the resulting non-toy is pretty cool. The extruders on my Replicator Dual are not attractive to the magnets, but if they were this print would not be possible. You should check your own extruders just in case.