Pictured in the upper left corner is a basic two inch (5 cm) cube which can be created without glue from six standard size business cards. These basic cubes, by interlocking the flaps on the sides, can be connected together to form larger constructions. On the right is a letter "Z" built by Sam Allen using 2,400 business cards. In the lower left corner is a depth three Menger’s Sponge built by Doctor Jeannine Mosely using 66,048 business cards! You will notice that the surface of the Menger's Sponge is flapless compared the surface of the "Z". A flapless cube may be constructed by adding business cards with the flaps turned inward to each of the six faces of the basic cube.Excellent illustrated instructions by Ned Batchelder on how to build the basic cube as shown in the upper left can be found here. Details on the construction of the "Z" can be found here. And finally, the story on the Menger Sponge construction can be found at The Institute for Figuring website which also offers instructions here that illustrate nicely how to create flapless cubes.
