Why do books have dog ears? I googled the question and found no explanation. It seems that dog ears are inevitable (for paperbacks, notably) even if you've always been careful. From my experience, they are about equally likely to appear on the top corners as on the bottom corners (for both the beginning pages and the ending ones). Dog ears for the middle pages of the book are less likely but they can also appear in frequently used old books. Can someone explain why? 


I apologize if this is not the right kind of question to post here. I can find no other sites on SE for it. 
 A: I think this is primarily about plastic (non-reversible) deformation.  Plastic deformation appears when the stress of material is large enough.  Stress is a generally speaking ratio between "force" and "dimension" of the object.  At the corners, this ratio is larger, even for the same force, as "dimension" is smaller, that is paper tends to get narrower toward the corner.
Imagine that you want to fold paper through the center or at the very edge.  A much larger force will be needed in the former case as the dimension of the fold is much larger.
Part of the problem might be also, that you are usually turning pages by pulling page by its corners, so you usually apply force close to future dog ears.
A: Paper is made out of cellulose fibers which are bound together by small amounts of glue under significant heat and pressure in the paper mill. Under a scanning electron microscope the surface of a sheet of paper looks like a haystack that has been smashed flat by great pressure.
As long as the glue (called binder) retains its grip on the deformed cellulose fibers, the paper sheet is stable i.e., its dimensions (including its thickness!) do not change. But mechanical flexure gradually breaks down the structure of the paper sheet and the squashed cellulose fibers loosen up and want to relax back to their unsquashed state.
In a book, the portion of the sheets that get flexed the most are the corners, which slowly grow in thickness as the cellulose comes unglued. Moisture and finger grease make this swelling worse, and high quality paper resists this better than low grade paper.
A: Alright, your picture made me understand what you're talking about.
Its the moisture that's doing this. Paper is hygroscopic--it absorbs water from the atmosphere, if only a little bit.
Now, paper is wood (pulp). And wood contains plant cells. Plant cells(or whatever's left of 'em) absorb this water and swell. This causes the paper to "warp" (you may have noticed this while using watercolors--or just dunk a scrap of paper in water and see what happens when it dries). This warping is due to the fact that the paper has limited area, so the molecules have nowhere to expand but up/down.
Now, if you have a bunch of sheets of paper, the warping will not be the same for each one--as in the warped "humps" will not necessarily fit into the warped "troughs" and vice versa. Since the sheets no longer have a snug fit, we have a lot of extra space. This causes the book to "puff up". The corners have more freedom than the rest of the page, so they can at least curl away when puffing up.
I once got a book (physics book, incidentally :/ ) wet in the rain when I'd kept it in the non-waterproof section of my bag. It puffed up like crazy. After drying it and flattening it, it's no longer puffed up as much, but the edge of the book is still pretty U-shaped.
A: The dog ears on books probably result from the moisture in our hands. I have a Bible that is dogeared particularly in the lower right corner. The edges are also dirty. This is so even though I never deliberately dog ear my Bible and my hands appear clean when I read it. The dog ear starts as a curl in the corner and sometimes eventually becomes an actual fold. I try to un-dog ear the corners by folding them back, but I might be exacerbating the problem by adding additional moisture from my fingers to the corners. When I was a child, a librarian taught me to turn pages from the upper right hand corner. Perhaps this is because books tend to be dog eared most from the lower right hand corner. I am going to do an experiment and buy a new Bible and read it only with gloves on to see if it gets dog eared. Of course, this experiment will take a long time, at least a year, to see results. 
A: Perhaps this moisture absorption and curling problem is influenced and exaggerated by small amounts of salt from the fingers which tends to increase the absorption of water because it is so hygroscopic. We all  have salt  on our skins from perspiration, even in very small quantities, although it's much exaggerated in warm weather. The salt accumulates on the corners of books a we read and handle them over time. Perhaps someone needs to experiment with this, purposly applying small amounts of salt an∂ moisture to the corners of a paperback. One corner could have only natural perspiration applied. the other  nothing. A second book could further test the idea by having a paperback applied a more concentrated salt / water  solution of moisture  on one corner with the second corner again used as a control. Does salt concentration make a difference?   A third book could be used to test the upper / lower corner difference by having the same concentration applied to both corners. The real question here is do we apply more salt to the lower or upper corners of the book.
Sounds like an interesting experiment for someone who has the time, etc. Joe.
