See the notes for the net of a cube to see how to print this net and make your own tetrahedron.
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This model (right) was made from a kit with magnetic connections. Since it just shows the edges, you can see through the model, which means that you can count the vertices and edges easier. How many vertices (corners) and edges are there? See Euler's formula.
Tetrahedra are not very common. They do have one useful property; they are very stable. A caltrop is an unpleasant medieval weapon. It has four sharp points, arranged at the vertices (corners) of a tetrahedron. Whichever way you throw it, one point will always point straight up. Anyone treading on this will get a spike through their foot!
You can get tetrahedra packaging, usually for liquids such as fruit juice. These are made in a clever way, which you can try for yourself. Make a cylinder of paper and glue the edge down. Pinch one end, and glue that. Now pinch the other end in the opposite direction, and glue that. It will naturally form a tetrahedron, although you might need to play around with the dimensions of the cylinder to get a regular tetrahedron.
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