What is the mechanism of magnetic core saturation?

Why does a magnetic core saturate? What is its actual mechanism?

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@ths1104: Once saturated, further incremental increases in the applied field strength $\Delta H$ result in increases in the magnetic flux density $\Delta B$ that are proportional to the vacuum permeability $\mu_0$: $\Delta B = \mu_0 \Delta H$. –  Art Brown Dec 17 '12 at 18:34