People rarely move at random. Geographers model human migration with a gravity model borrowed from physics: the flow between two places grows with their size and fades with the square of the distance between them — big, close places pull hardest.
Every place has push factors that send people away (drought, few jobs) and pull factors that draw them in (work, safety, family). Raise the push and pull dials and watch migrants re-sort themselves across the map until the flows balance.
When inflow and outflow match everywhere, the map reaches equilibrium — a stable distribution. Grow one city and its pull can snowball into a dominant magnet.
Drop a drought on a region and its push spikes: people pour out in an exodus, rerouting the whole map — exactly how a single shock reshapes where a population lives.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.