An icosahedron is a three-dimensional shape with twenty flat faces, each one a triangle. If you've ever played tabletop games like Dungeons & Dragons, you've probably seen an icosahedron: it's that cool twenty-sided die that players roll to determine success or failure in the game.

Each face connects to its neighbors along the edges, and if you count carefully, you'll find twelve corners where the triangular faces meet. The regular icosahedron, where all the triangular faces are identical, is one of only five Platonic solids, special shapes known since ancient Greece where every face, edge, and angle is exactly the same.

Icosahedrons appear in nature too. Many viruses have icosahedral shapes because this geometry uses the least amount of material to enclose space, making it an efficient design. Geodesic domes, like the famous Spaceship Earth at Disney's EPCOT, are based on icosahedral geometry, using triangular panels to create strong, spherical structures.

You might build an icosahedron in math class as a paper model, carefully folding and taping twenty triangles together. It's trickier than making a cube, but when you finish, you'll have created one of geometry's most elegant shapes.