A crystalline nanoparticle's surface is not chemically uniform. Most surface atoms sit on flat terraces with the maximum coordination a surface position allows, making them relatively unreactive. Step edges and kink sites have fewer bonded neighbours — they are under-coordinated — which leaves bonding capacity available to break apart incoming molecules far more readily. This simulator builds two nanoparticle shapes with the same total surface-atom count — a smooth faceted crystal with few edges and a rough stepped crystal with many — colours every surface atom by its coordination class, and lets reactant molecules land at random across the surface so you can watch terrace landings sit inert while step-edge and kink landings react almost immediately, with a live reaction-rate comparison between the two shapes.