This learning resource focuses on classifying major carbon reservoirs on Earth and understanding their relative storage capacities. In Part I, students will categorize various carbon components, such as atmospheric CO2 gas, terrestrial vegetation, and fossil fuels, into four Earth spheres: Atmosphere, Biosphere, Hydrosphere, and Geosphere. They will also rank these reservoirs according to their storage capacity based on geological evidence. Part II emphasizes analyzing reservoir dynamics through metacognitive calibration by predicting which reservoir can sequester carbon for the longest time. Students will compare the high-capacity Geosphere to the low-capacity Atmosphere by examining the characteristics that influence carbon's residence time. Lastly, there is a discussion on why certain reservoirs act as carbon sinks versus those that do not.