*Schools should use open-source textbooks rather than traditional textbooks.* That’s the case David Thornburg makes in a recent blog post on *Edutopia*. Dr. Thornburg bases his reasoning on last month’s discovery by NASA that a microorganism living in Mono Lake uses arsenic rather than phosphorous to construct its DNA. He is entirely right to point out that this discovery makes all life-science textbooks incorrect, because these textbooks state that carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur are the six basic building blocks of all known forms of life on Earth. Uhhh, not so much anymore.

I would make a case slightly differently than Dr. Thornburg’s argument, as follows: Any textbook or curriculum that treats learning as the memorization of facts, skills, and routines should be abandoned, regardless of whether it is delivered digitally, in hard copy, through an open-source portal, or through a commercial publisher. Rather, learning should be defined by the habits of mind that students will be able to employ long after they have completed their formal education. Students will encounter changing facts throughout their lives as new scientific discoveries are made. Schooling should prepare them to question, accept or reject, and place into context new findings such as this recent discovery by NASA. Students are entitled to curricula that embody, and teachers who support, this type of learning.

For instance, if I asked my former students, now in their 20s and 30s, how many possible diagonals can be drawn within a convex nonagon, I would not expect, or be disappointed, if they did not remember the formula *# of distinct diagonals = n(n-3)/2*. I would, however, be very disappointed if they were unable to derive the answer by drawing a number of polygons and making a reasoned conjecture of what the answer will be.

On the same grounds that reasoning, justification, sense making, and constructing arguments trump facts and skills in a curriculum, I would argue that the Standards for Mathematical Practice take precedence over the Mathematical Content Standards. We are preparing students for a 21^{st}-century world in which foundations can shift, as they just did for the basis of life on Earth; to help students move forward into adulthood, we must help them develop the ability to accept and work with new information. As we evaluate standards, curricula, teaching, and educational software, we should ask not only, “*Is it up to date? Is it affordable?” * but also, “*Does it ask my students to reason, justify, model, make sense, and construct?”*