Here's why:

* Body fossils are the actual preserved remains of an organism, like bones, teeth, shells, or even the entire organism itself.

* These remains can be preserved in a variety of ways, including:

* Impression: A mold of the organism's body, like a footprint in soft mud that later hardened.

* Petrification: The organism's tissues are replaced by minerals.

* Mummification: The organism is dried out and preserved in a dry environment.

* Amber: The organism is trapped in fossilized tree resin.

So, when we talk about a "body impression", we're referring to a type of body fossil where the organism's shape is preserved as a mold in the surrounding rock. This is often seen with soft-bodied organisms that don't readily fossilize in other ways.