Is it alluminium? No, it's...

And the answer to the previous riddle is...
Wood.

Or, more exactly speaking, its cell wall, half of which is cellulose (the same fiber which paper is made of).

Consider this: the tensile strength of these wood cell walls is about 150 MPa (each square centimeter stands 1.5 tons!); the tensile strength of alluminium ranges from 25 to 125 MPa. Thus, if we had two rods of the same size, one made of alluminium and one made of pure and perfect wood, the wood one would break at a higher force!

Remember what I told you in a previous post: wood was developed to hold up the tree and to carry the sap from the roots to the branches. So the idea was to develop a very strong material. Not only to use less amount of it, but also because the more holes it has, the more sap that arrives to the right place. More with less.

Material properties are taken from Ashby and Jones. "Engineering Materials 2. An Introduction to Microstructures, Processing and Design" 3rd edition. Elsevier, 2006.