Cast fossils form when a mold is filled in. Once a hollow mold is present, they are filled in with minerals that form a solid.
Mold fossils:
Mold fossils are an imprint mark made in a rock or sediment. They are hollow, and are often leaves, teeth, claws, and more.
True Form fossils:
True Form fossils are big parts of an organism went through a process called petrification, in which their body part was replaced by minerals which hardened to become rock.
Any living or non-living thing trying to take flight will have two forces working against them: gravity and drag. Gravity is the force that the Earth exerts on all things pulling them back towards its centre. ... Birds also have two forces that help them in their attempts to take flight.