Earth's mantle is solid rock, but over millions of years it flows like a slow fluid. Heat leaking out of the core keeps the bottom hot: hot rock is less dense, so it rises; at the cold top it spreads, cools, grows heavy and sinks. That loop is a convection cell — the heat engine of Class 2.
The rigid plates are just the chilled skin riding on top. Where the mantle rises it drags them apart and new crust wells into the gap — a mid-ocean ridge. Where it sinks it pulls a plate down into a trench. Divergent and convergent boundaries aren't drawn on a map; they're wherever the conveyor reaches the surface.
This model is a real 2-D fluid solve (Boussinesq convection). Its Prandtl number is near 1 for stability, not Earth's enormous value, and time is compressed from millions of years to seconds — but the onset of convection, the cells, and faster-flow-from-more-heat are all honest.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.