In first or second grade, I believe I was asked to "show my work" for "2 x 3 = ?". I believe the teacher didn't accept "But I don't have any work to show" as an answer. I don't think most adults would have any work to show either, nor should they—if they actually repeatedly add 2 to itself, or do anything other than a memory lookup, then there's probably something wrong with their prior math education.

In general, if the step that I took to solve a problem was "I looked up a memorized fact in my brain", then that is the truth, and writing down anything else to "show my work" is a lie.

As other people have said, a proper way to ask that question is "Show a derivation of the answer to [problem]." Bonus points if you specify what assumptions they're allowed (e.g., in the above example, "addition").