This lovely photo by my fellow GeekDad writer Nathan Barry shows what you can do when you understand physics, and forces, and all that stuff. As Wikipedia explains:
A cantilever bridge is a bridge built using cantilevers, structures that project horizontally into space, supported on only one end. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete. The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can span distances of over 1,500 feet (460 m), and can be more easily constructed at difficult crossings by virtue of using little or no falsework.Everything I know about bridges I learned from the Building Big book/TV series/website by David Macaulay. Here's how he explains cantilever bridges, using the Firth of Forth bridge as an example:
Thanks to Teacher's Domain for the clip!