Pick a network topology and see it drawn, with the three numbers architects actually compare: degree (links per node), diameter (worst-case hops), and bisection (links cut to split it in half). Slide the size and watch how they grow.
Try this: start on Mesh and slide the size up — watch the diameter climb. Switch to Torus at the same size: same nodes, smaller diameter (the wrap-around paid off). Then Crossbar: perfect diameter, but look at the switch count explode as N².