When is a door not a door? A Common Core math riddle

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A frustrated parent posts a photocopy of a page from a math workbook at the Facebook page Common Crud, along with a verbal “sigh”:

This is part of tonight’s 2nd grade math homework. I’m not sure how a second grader is supposed to have any clue why the flat surfaces on a rectangular prism are called faces. I don’t know why. And the answer was nowhere to be found on the paper.

Why-are-surfaces-called-faces

According to the category heading that precedes the problem, the common core kiddies are supposed to arrive at an answer by means of the skill of reasoning. Come, let us reason together.

Putting aside entirely the appropriateness of this question for Grade 2, how is this a math question and not, say, one of etymology or philosophy? A search for answers, in any case, yields a couple of possibilities.

A commenter at the open-ended website WikiAnswers writes:

The flat surfaces of all polyhedra are called faces and a rectangular prism is simply one kind of polyhedron.

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