Given a context passage, generate a question from the passage such that its answer is shortest continous span from the passage.

At the planets center lies a dense metallic core. Scientists know that the core is metal because: 1. The density of Earths surface layers is much less than the overall density of the planet, as calculated from the planets rotation. If the surface layers are less dense than average, then the interior must be denser than average. Calculations indicate that the core is about 85% iron metal with nickel metal making up much of the remaining 15%. 2. Metallic meteorites are thought to be representative of the core. The 85% iron/15% nickel calculation above is also seen in metallic meteorites (Figure 1.1). If Earths core were not metal, the planet would not have a magnetic field. Metals such as iron are magnetic, but rock, which makes up the mantle and crust, is not. Scientists know that the outer core is liquid and the inner core is solid because: 1. S-waves do not go through the outer core. 2. The strong magnetic field is caused by convection in the liquid outer core. Convection currents in the outer core are due to heat from the even hotter inner core. The heat that keeps the outer core from solidifying is produced by the breakdown of radioactive elements in the inner core. Click image to the left or use the URL below. URL:
scientists know which layer of the core is liquid and which is solid because