Have you ever thrown an apple core or an orange peel out of the car window? Is this littering? What happens if it lands on the pavement? Is it the same as if it lands in a patch of soil on the side of the road? Apple cores, orange peels, leaves, and other plant materials are made up of complex carbon compounds, which get broken down over time into simpler forms of carbon in a process called decomposition. Decomposition is a key part of the carbon cycle. And decomposition is largely done by microbial communities, groups of tiny living organisms, invisible cities that live everywhere on Earth. Can you imagine these invisible cities, eating the apple core that you threw away or a pile of leaves on the ground? How quickly do the dead plants decompose? And where is the carbon going?