In order to qualify for protection, a work must be an expression with a degree of originality, and it must be in a fixed medium, such as written down on paper or recorded digitally. The idea itself is not protected. That is, a copy of someone else's original idea is not infringing unless <answer>it copies that person's unique, tangible expression of the idea</answer>. Some of these limitations, especially regarding what qualifies as original, are embodied only in case law (judicial precedent), rather than in statutes.
What makes a copy of an original idea infringing?
it copies that person's unique, tangible expression of the idea