I googled the question and found no explanation. It seems that dog ears are inev

Question

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?

dog ears on the bottom corners for the beginning and ending pages

dog ears for the beginning pages of a book

I apologize if this is not the right kind of question to post here. I can find no other sites on SE for it.

Explanation / Answer

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.

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