This activity is meant to explain some of the science behind how geologists use data from seismometers around the world as evidence of the Earth’s solid core. Earthquakes occur within the Earth’s crust. Keep in mind that if the Earth were an apple, the crust is about the thickness of the apple skin, so for all intents and purposes, the epicenter of an earthquake occurs at the surface.
Vibrations recorded on seismometers are greatest above the epicenter (the point in the crust, below the surface, where the vibrations originate), but for large quakes, they can be felt around the world. Curiously though, directly across form the epicenter, and for some distance to each side, the vibrations are absent or much reduced. Refraction would bring some vibrations everywhere. It is hypothesized that this vibration “shadow” is caused by the solid core of the Earth absorbing or reflecting the vibrations.
Vibrations are able to travel more easily through the liquid mantle, but do not travel easily through the solid core.
For older students, you can extend this activity by considering it in three dimensions. The area across from an earthquake where the vibrations are not felt as strongly as expected (if the Earth were liquid all the way through) is actually a circle, and the “shadow” forms a truncated cone.