In this lesson, students examine seismic evidence to determine that the Earth must have a layered internal structure and to estimate the size of Earth’s core. Students are divided into two teams (theoreticians and seismologists) to test the simplest hypothesis for what is inside of the Earth: that the Earth is homogeneous throughout. The theoretician group uses a scale model of a homogeneous Earth combined with average seismic wave velocity to make predictions about when seismic waves should arrive at various points around Earth (predicted). The seismologist group interprets seismic data from a recent earthquake to determine when seismic waves actually arrive at various points around Earth (observations). Students then compare and discuss the fit of the predicted and observed data and then use a second scale model to further interpret these results. The students then measure the Earth’s outer core based on their data.