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"Tetra-" is the Greek root word for the number 4. "Tetrahedron" is the name given to 3-dimensional structures that are created by assembling 4 triangles into a single enclosure. One triangle becomes the base, and the other three are attached to it, pointing upward and leaning toward each other to form a peak, opposite the triangular base.

Various types of drawings can be used to represent tetrahedrons. Here are two examples, using opaque walls.
Here are two more examples, using transparent walls (or, using lines, sticks, or pipes to represent the edges, with open spaces rather than walls in the triangles that are formed when these types of lines, sticks, or pipes are connected to each other in this manner).

Notice that these are similar to Egyptian-type pyramids; however, Egyptian pyramids have square bases, while tetrahedrons have an additional triangle, instead, as the base. Among other things, this means that tetrahedrons have more types, layers, and levels of symmetry, than pyramids; if you knock over a tetrahedron, so that it rests on one side, it simply becomes the exact same shape as before.

Also notice that tetrahedrons do NOT need to be made from equilateral triangles; instead, for example, a lower or upper corner, in a rectangular room, graph, or chart, can be regarded as a tetrahedral shape (i.e., composed of 4 triangles, all connected together to form a closed structure or geometric shape), as indicated in this drawing:

While you're at it, please notice that the rotating figure, below, becomes a perfect square or diamond shape, at one point (actually, two points) during its rotation. And please notice, also, the cross that appears in the middle, at those moments when it looks like a square or diamond.
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