How do you recognize an "A" paper?
For me, there are two kinds of "A" projects - those that score in the low 90's, and those that score in the high 90's. The first kind is attained by fulfilling all the expectations outlined in the rubric - usually a combination of following directions, accurate & complete science content, no/few spelling & grammar mistakes, and an attempt at creativity, originality, or style. This is the "I did everything you asked me to" kind of "A," and it should not be looked down upon, for it is achievable by any student who puts his or her mind to it. Nevertheless, these papers do not always seem special - they can, at times, come across as a bit mechanical.
The second kind of "A" project gets a score in the high 90's. It meets all the requirements of the rubric, does everything I asked, crosses t's and dots i's. And then it goes beyond what I asked for, and shows signs that the student truly "got" the point of the assignment. Sometimes, it might incorporate skills which I only hinted at, but did not explicitly teach. For sixth graders, an example of this is a research paper that is not only complete, but is organized so that the information flows logically from one topic to the next - a skill that is hard to teach and which I have not had much success developing across the board. Sometimes, this kind of project conveys a passion for the subject matter which I could not, by any means, require of the students, but which makes the project so much more exciting than the average. In almost all cases, it is work that shows the student pushed himself or herself, which goes beyond what the student has previously achieved. There are a few students who consistently turn in this level of work, and many others who produce high-"A" work from time to time, and many for whom it is a rare achievement.
I must also note that I allow students to re-do any project scoring lower than 75%. Once in a great while, usually with some extra help between the original and the re-do, a failing project turns into an "A." The smile on the student's face who realizes he or she has just produced top-quality work? Priceless.