Here's why:
* Metaphor: A figure of speech where a word or phrase is applied to an object or action to which it is not literally applicable.
* Literal Meaning: A bone is a hard, rigid part of the body, and contention is a disagreement or argument.
* Figurative Meaning: The phrase "bone of contention" refers to a subject or issue that causes strong disagreement or argument.
The phrase compares the source of the disagreement to a literal bone, which is something people might fight over.