Cardboard is made from pine trees. Trees are debarked by passing through a ring-type debarker, a rotating ring of knives like a giant pencil sharpener. Debarked trees are then chipped. The chips go into a pressurized tank to undergo the kraft process, which makes pulp. In this process, used in approximately 80 percent of paper production, chemicals break down the lignin, the organic substance that binds plant cells and, in trees, holds the wood fibers together. The resulting wood pulp is pressed into rolls of paper.
Cardboard consists of three layers of paper--a corrugated sheet (the medium) between two flat sheets (liners)--held together with starch.
Considering that it consists only of three layers of paper, cardboard is strong, due to the corrugated material's fluted middle layer that lends strength. Stacked boxes, with the flutes lining up in the vertical dimension, are especially strong.