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In this popular YouTube video, a man attempts to fold a piece of paper more than seven times using a hydraulic press.

After the seventh fold was applied pressure from the press, the man shows the audience that the paper became brittle, and breaks apart almost how styrofoam does.

Paper isn't usually brittle, so what exactly took place such that the material properties of the piece of paper wildly changed?

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In short, Paper is made from wood and it has properties of wood. While in thin sheet of paper we cannot feel significant difference. but when many papers are combined and pressed(see the book video on same channel), it behaves more like wood. but due to cutting into thin sheets it will not retain same strength of wood.

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If you do it right, paper may be folded in half more than seven times: http://mathworld.wolfram.com/Folding.html

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When folding paper multiple times the radius of each layer at the fold is different. By applying pressure to the folded structure outer layers are subjected to a higher tensile force, than inner layers. The thicker the fold, the larger the radius, the more tensile stress on the outer layer. It happened to be that it ruptured (I.e. the stress in the outer layers became larger than ultimate tensile stress for the paper) on the 7th fold in this case. But overall apparent "brittleness" isn't related to folding at all. This is essentially a composite material formed from multiple layers of highly compressed paper. The same effect would have been if the layers were pre-cut instead of folded.

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