It is... like what?

As a way of celebrating Earth Day, choose some part of nature and describe it by saying that it's like something else. Or that it is something else.

A Shell

is a shape. It is a shade. A seal, broken. An equivalent of a bone. A home of someone who has left. A spiral. A staircase. One half of a two-part whole. It is a hole and a corridor. A cradle. A microbowl. A pink nail left in the sand. A white shard buried in a dune. A raven's beak's crescent moon. A stamp on the surface of a stone. A necessary stage. A story, lost and found.