Tetrahedron

The tetrahedron, with four equilateral faces, is the simplest and sharpest of the regular solids, and in the Timaeus it is the shape of fire. Its small size and acute angles explain fire’s piercing, cutting, mobile nature — the most penetrating of the bodies.

Like air (octahedron) and water (icosahedron), it is built from the half-equilateral triangle, so these three can dissolve into and recompose one another; only earth, built from a different triangle, stands apart. See the overview of the Platonic solids.