A tetrahedron is a geometrical shape made from four equilateral triangles. An equilateral triangle is a three-sided shape of which all three sides are of equal length. The word tetrahedron means "with four planes" and the shapes are always three-dimensional figures. Architects and crafters can also string and group tetrahedrons together to make interesting structures and patterns. The mathematics behind a tetrahedron can be daunting, but it is simple to create a small-scale tetrahedron model using basic household objects.
Fold the envelope in half from left to right and press down so that you crease the middle of the envelope vertically. Open the envelope again, laying it flat on the table.
Angle your ruler over the envelope diagonally so that the ruler spans from the upper left corner of the envelope to the lower right corner. Trace the line, pressing hard along the edge of the ruler with your pen so that you leave an indentation where the line is drawn.
Angle your ruler over the envelope the opposite way, so that it reaches diagonally from the top right corner to the bottom left corner. Press with your pen along the ruler's edge so that you make an indentation along the line. This will make a large "x" in the middle of your envelope.
Cut the flap along the lines of the top portion of the envelope so that the remainder of the envelope has a "v" across the top. Discard the top portion of the envelope that you have cut out.
Open up the envelope and fold it along the vertical crease that you made in the beginning of the procedure. Then, push the upper left corner of your envelope until it is inside of the lower right corner. Smooth out the shape and double check to make sure that the folds are neatly tucked in.