Expanded clay is a lightweight aggregate made from natural clay that is heated in a rotary kiln. During this process, the clay expands and forms small, round pellets that are porous and have a low density. These pellets are often used in construction, gardening, and hydroponics due to their excellent drainage and aeration properties. In gardening, expanded clay is popular as a growing medium because it helps retain moisture while allowing excess water to drain away. It is also used in green roofs and landscaping to improve soil structure and reduce weight on structures.