At a mid-ocean ridge, molten rock rises, cools, and hardens into brand-new ocean floor. As it freezes, iron minerals lock in the direction of Earth's magnetic field like tiny compass needles — a permanent recording.
Earth's magnetic field reverses at irregular intervals: north and south trade places. Each reversal makes the next band of new crust point the opposite way, so the seafloor becomes a striped barcode of normal and reversed rock.
Because crust is added to both sides of the ridge at once, the barcode on the left is a mirror image of the one on the right. Ships towing magnetometers found exactly that symmetry in the 1960s.
That matching, mirror-imaged pattern — the Vine–Matthews–Morley discovery — was the evidence that finally proved the seafloor spreads and the plates move. Wide bands mean fast spreading or a long-steady field; the crust at the far edge is the oldest of all.
Something in the simulation stopped unexpectedly — the lesson continues without it. Nothing you did was wrong; you can move on.