Eli5 What scientific properties make paper bend & crease?
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On a microscopic level, paper is cellulose fibers all piled up on each other. These microscopic fibers are fairly elastic, they can bend and then go back to how they were before. If you bend them too far, though, they will break. You might have noticed that paper is much easier to tear along a crease, or that something like a map that has been folded and unfolded many times will start to develop breaks along fold lines. This is basically because whenever you form a crease, some of the fibers are broken at the crease line. They broken ends of the fibers are wedged into the others, though, but they want to maintain their shape, which is what maintains the crease line. If you fold and unfold the same line enough though, the crease loses all strength as those broken ends get broken down even more – this is why you have to start Origami or paper airplanes with fresh, flat paper if you want it to come out rigid
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