Final answer:
The densest metals sunk towards the central core of the Earth, while lighter silicates rose towards the surface. These processes occurred due to the pull of gravity and the planets being hot enough to melt at one point.
Step-by-step explanation:
When we look at the internal structure of each of the terrestrial planets, we find that the densest metals are in a central core, with the lighter silicates near the surface. If these planets were liquid, like the giant planets, we could understand this effect as the result of the sinking of heavier elements due to the pull of gravity. This leads us to conclude that, although the terrestrial planets are solid today, at one time they must have been hot enough to melt.