Not that I'd ever put it in those terms to a student, but, really, in copyright: you have to read the statute. For reasons not worth exploring at this juncture, I'm reading the briefs in Garcia v. Google, the case brought by the actress in the Innocence of Muslims case where she claims to own the copyright in ... something. Her performance as fixed in the film, is the best way to say it. One of Google's arguments is that, under the CCNV test, she was an employee working within the scope of her employment, and her lawyers' outraged response is that there was no written agreement making her contribution a work for hire.
17 U.S.C. 101: A “work made for hire” is—
a work specially ordered or commissioned
... if the parties expressly agree in a
written instrument signed by them that the work shall be considered a
work made for hire.
While I express no opinion on the merits of Google's analysis of whether Garcia was an employee using the multifactor CCNV test, Garcia's claim would be more credible if Garcia's lawyers spent less time being wrong.