Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Earth's mantle, primarily composed of silicate and magnesium oxide rocks, floats on top of the liquid iron and nickel outer core. Which difference between these two layers of Earth resulted in their differentiation?
Density.