Earth is not the same all the way through. It is made of three main layers: the crust (the thin, rocky outer shell we live on), the mantle (a thick layer of hot, slowly moving rock beneath the crust), and the core (the dense center made mostly of iron and nickel, with a liquid outer core and a solid inner core). Each layer has different thickness, temperature, and composition. The crust is thinnest (5-70 km), the mantle is thickest (about 2,900 km), and the core is about 3,400 km in radius. Temperature and pressure increase with depth.
Cut a hard-boiled egg in half to show the three layers — shell (crust), white (mantle), yolk (core). While the proportions are not exact, the analogy of concentric layers is powerful. A peach works too: skin, flesh, pit. Then show a scaled diagram where students can see how incredibly thin the crust is compared to the rest of the planet. Discuss how we know about layers we have never seen — seismic waves from earthquakes travel differently through different materials.
If you could slice Earth in half like an apple, you would see that it is built in layers, like an onion. Three main layers make up our planet, each very different from the others.
The outermost layer is the crust — the thin, solid shell of rock that we live on. It is incredibly thin compared to the rest of Earth. Under the continents, the crust is about 30-70 kilometers thick. Under the oceans, it is even thinner — only about 5-10 kilometers. If Earth were the size of a basketball, the crust would be thinner than a sheet of paper wrapped around it. Despite being so thin, the crust is all we can directly access. The deepest hole ever drilled went about 12 kilometers — not even through the continental crust.
Below the crust is the mantle, which makes up about 84% of Earth's volume. The mantle is roughly 2,900 kilometers thick and made of dense, hot rock. A common misconception is that the mantle is liquid lava, but it is actually solid — just extremely hot (temperatures reach about 4,000 degrees Celsius near the bottom). Under the enormous pressures deep inside Earth, this hot rock behaves like very thick, slow-moving putty. Over millions of years, it flows in giant convection currents: hotter rock rises slowly, cooler rock sinks. This slow churning is the engine that drives plate tectonics, earthquakes, and volcanic activity at the surface.
At the center is the core, made mostly of iron and nickel. It has two distinct parts. The outer core is liquid metal — so hot (4,000-6,000 degrees Celsius) that the iron and nickel have melted. This swirling liquid metal generates Earth's magnetic field, which protects us from harmful solar radiation. The inner core is even hotter (up to 6,000 degrees Celsius — as hot as the surface of the Sun), but the pressure is so enormous that the metal is squeezed into a solid ball about 2,400 kilometers across. The fact that we know all of this without ever seeing it directly is remarkable — scientists mapped Earth's interior by studying how earthquake waves travel through the planet, changing speed and direction at each layer boundary.