The mass of the Earth has been calculated to be 5.98 × 1023 kg. The scale of this measurement is difficult to comprehend and impossible to measure directly. In this activity, students will use water displacement to make smaller scale measurements of the density of rocks. Averaging and comparing the results gives students insight into how the density and mass of the Earth are calculated. An excellent activity to teach students about the layers of the Earth and to reinforce important math skills. Includes reproducible student handouts and valuable Teacher Notes.