600-cell

In geometry, the 600-cell (or hexacosichoron) is the convex regular 4-polytope, or polychoron, with Schläfli symbol {3,3,5}. Its boundary is composed of 600 tetrahedral cells with 20 meeting at each vertex. Together they form 1200 triangular faces, 720 edges, and 120 vertices. The edges form 72 flat regular decagons. Each vertex of the 600-cell is a vertex of six such decagons.