Planetary Differentiation
Planetary Differentiation is how a planet sorts itself so heavy stuff sinks to the center and light stuff rises to the top.
Think of it this way
Oil and water separate in a clear jar. The heavy water sinks to the bottom, while the light oil floats to the top. A planet does the same thing when it is young and molten. Heavy iron sinks to form the core, and light rock rises to form the crust.
Explaining planetary differentiation by grade level
Think about a jar of muddy water. If you let it sit, the heavy sand falls to the bottom and the light water stays on top. A new planet works the same way. When it was very hot, the heavy metal sank to the middle and the lighter rock floated up around it.