a bone of contention

Example: “Who gets the window seat is a constant bone of contention between my kids.”

Origin: From dogs, and the older English phrasing makes it plainer than the modern one does. To cast a bone between was to throw something down deliberately in order to provoke a fight, and English also used bone of dissension alongside the surviving form; both are attested from the 1500s. The image depends on a specific property of a bone rather than food generally — it cannot be shared or divided usefully, so two animals presented with one have no available outcome except contest. That is what distinguishes a bone of contention from an ordinary disagreement: it names not just a topic people argue about but one structured so that agreement is unavailable, since whatever satisfies one party necessarily denies the other.