A box and lid that make a cube outside and has a cube shaped space inside and a regular tetrahedron whose edges are the same length as the diagonal of any face of the inside cube space. The tetrahedron fits inside the box when one of its edges lays diagonally on the bottom of the box. This demonstrates that a regular tetrahedron's vertices can be defined by the two end points of two diagonals on opposing sides of a cube with one diagonal rotated 90 degrees around an axis defined by the center points of the diagonals. (There's probably a better way to say that!) The length of an edge of the tetrahedron is equal to the length of a diagonal of a face of the inside cube space in the box. The tetrahedron and the cube are the second and third simplest regular solids (a.k.a., 'Plutonic' solids). The sphere is the simplest regular solid, and the tetrahedron is the simplest regular solid with plane sides. This also demonstrates that a regular tetrahedron can be "parallel projected" (viewing along the axis formed by the center points of any two opposing edges) to form a square. Added a stellated octahedron to this Thing. The stellated octahedron, a.k.a, "stella octangula", may be thought of as two tetrahedrons combined, with one tetrahedron rotated 90 degrees from the other on any one of the central axes. This stellated octahedron uses two combined tetrahedrons of the same size as the tetrahedrons in this Thing, and therefore, it too fits perfectly into the box.Drawn in SolidWorks, printed on BFB 3D Touch using 0.25mm layers. The tetrahedron is hollow with 2mm thick walls. The cube also has 2mm thick walls. If I try to print a single tetrahedron, the top point prints too rapidly and the plastic doesn't have time to cool between layers and the point gets distorted. One way to stop this is to print two or three tetrahedrons at the same time -- the plastic then has time to cool down enough as it prints the alternate parts. Note that printing the "All three parts" file doesn't solve this problem because the top point of the tetrahedron is higher than anything else in the print. So it is best to print the parts separately and print 2 or 3 tetrahedrons at once.