Here's why:
* Mold Fossil: A mold fossil forms when an organism is buried in sediment, and then the organism decays or dissolves, leaving behind an empty space that is the exact shape of the organism. This empty space, the mold, preserves the organism's external features.
* Cast Fossil: A cast fossil forms when a mold is later filled with minerals or sediment. This filling hardens, creating a three-dimensional replica of the original organism.
So, if you find a hollow place shaped like an organism, it's most likely a mold fossil.
