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Turn an A4-sized letter into a folded square with the writing inside. Cool.

The unfoldable-ness of this box is not that it can't be folded, but that once it is made/folded and pulled into its box shape, it cannot be unfolded.

Interesting shape. Like a cross between a triangle and a hexagon.

Tutorial for an origami box. To make a lid, one uses slightly larger paper.