Bridges are interesting because they can hold a lot of weight, but they are not very heavy themselves.
Some bridges are mostly empty space.
Many bridge-like designs can be found in nature. Los puentes son interesantes porque pueden resistir mucho peso, aunque los puentes mismos no pesan tanto. Algunos puentes consisten de m s espacio vac o que de material. The kind of bridge we are building is called a truss bridge. Truss bridges are built out of triangles , which makes them very strong.