Platonic Solids

No, that's not a solid who's just a friend. :)  Named after Plato, who studied geometric solids.  He identified 5 shapes that could be formed from multiplying regular polygons (that is, a shape with equal angles and equal-length sides).
Here's a li'l octahedron (that's a solid composed of eight equilateral triangles) I made from Bristol board and love.