The tetrahedron is bounded by four equilateral triangles. It has the smallest volume for its surface and represents the property of dryness. It corresponds to fire.
The hexahedron is bounded by six squares. The hexadedron, standing firmly on its base, corresponds to the stable earth.
The octahedron is bounded by eight equilateral triangles. It rotates freely when held by two opposite vertices and corresponds to air.
The dodecahedron is bounded by twelve equilateral pentagons. It corresponds to ether and to the universe because the zodiac has twelve signs corresponding to the twelve faces of the dodecahedron.
The icosahedron is bounded by twenty equilateral triangles. It has the largest volume for its surface area and represents the property of wetness. The icosahedron corresponds to water.
With gracious thanks to Rüdiger Appel
http://www.3quarks.com/GIF-Animations/PlatonicSolids/