Why does folding paper make it stronger?
Short answer
Folding turns a flat sheet into a three-dimensional shape that resists bending. The paper itself does not suddenly become a stronger material; its new shape carries loads more effectively.
How it works
A flat sheet is very thin, so a push from above can make it bend or buckle. Folds create ridges and valleys, placing parts of the sheet farther from its center and making the overall shape harder to bend.
Corrugated cardboard and rolled paper use the same idea. The folds support one another and spread a load instead of allowing all of it to concentrate at one weak point. This lets a light material support more weight than a flat sheet.
Key point
Folding does not make paper stronger in every direction. The crease pattern, the paper thickness, and the direction of the load all matter. The same principle appears in origami, packaging, and lightweight structures.