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The Straight Dope 

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I am 10 years old and I ask my mom a lot of questions. She said to ask you because she asked you questions before and you know everything. My question is, why do gas stations sometimes have rocks on the roof? --Owen Kehoe, Seattle

You must be a pretty precocious kid, Owen, since you seem to have mastered computerized word processing, which has baffled a few 40-year-olds I know. I attribute this to the influence of your mother Joyce, with whom I have had a long and mutually entertaining relationship. Purely platonic, of course--but have her tell you the story about the Velcro nursing bra sometime.

To clarify your question, I assume you're not talking about rocks meaning boulders, but rather the layer of small stones you see not just on gas stations but on many flat roofs. Ideally the stones are river gravel, which has smooth edges and won't cut into the roof when somebody walks around on it.

Talk to ten roofing contractors and you'll get ten answers on what the gravel is for, but this seems to be the consensus: (1) Stabilization. In a typical tar-and-gravel roof (which, by the way, is gradually being supplanted by gravelless roof systems), you put down a layer of tar paper, then tar, and finally gravel. Tar has a low melting point. In the summer it gets pretty soft and if left to its own devices would flow to the lowest point on the roof. The stones force it to stay put. (2) Protection. The gravel prevents sun, hail, errant baseballs, etc from damaging the "membrane," the all-important moisture seal created by the tar paper and tar. (3) Ballast. The stones keep the tar paper in place and prevent it from buckling or blowing away in a high wind.

That's all there is to it. My regards to mom.

I was sitting here in my office grading students' papers and a question came to mind that only you can answer. Why is there A, B, C, D, and F, but no E? --Jim Ward, Austin, Texas

What makes you think A-B-C-D-F is standard? The University of Illinois, for one, uses A-B-C-D-E, E meaning failure. Admittedly most colleges use A-B-C-D-F, but at the elementary and high school levels, letter grading systems vary all over the lot. Cecil's research among his relatives has turned up the following: S (superior), E (excellent), G (good), F (fair), D (failure), public high school, 1942; A, B, C, X (conditional), D (failure), Catholic grade school, 1936; E, VG (very good), G, L (less than average), S (satisfactory for this child), U (unsatisfactory), Catholic grade school, 1966; E (working efficiently), C (not working efficiently but improving), N (not working efficiently), X (not marked in this grade), public grade school, 1938; E, G, F (fair), P (poor), public grade school, late 1960s.

A-B-C-D-F seems to be gradually supplanting other systems, particularly in high schools with college preparatory programs. Why E was left out nobody knows, but given that it means excellent in many systems, my guess is that some nameless pedagogue decided it would be too easy for a smart-aleck kid to convince his parents that a failing E grade was actually a high mark. I never had any failing grades, of course, but if I had, Lord knows I'd have tried it.

What was the "Great Vowel Shift" and why did it happen? PBS's Story of English series never explained it satisfactorily. --Sue, Detroit, Michigan

That's because there isn't any explanation. As Robert McCrum et al note in the book version of The Story of English, "phrases like the famous 'Great Vowel Shift' [are] hardly more informative than the 'unknown land' of early cartography." What happened was that between 1350 and 1550, the period in which Middle English became modern English, vowel pronunciations changed dramatically. The Middle English long i, formerly pronounced like the e in he, shifted to i as in high. Middle English hous, pronounced "hoose," changed to the modern house. The experts say the GVS "in effect moved the long stressed vowels forward in the mouth," but to me it just sounds like you open up your mouth more. Why it happened no one knows.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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