Molds and casts are two important concepts in the study of fossils. A mold is formed when an organism, like a dinosaur or plant, leaves an impression in sediment, creating a cavity that reflects its shape. This process often occurs when the organism decays, leaving behind a negative imprint. A cast, on the other hand, is created when minerals fill in the mold, hardening to form a replica of the original organism. This can provide valuable information about the structure and appearance of ancient life forms, helping scientists understand the history of Earth and its ecosystems.