La fortaleza de la calle cuatro: Una historia acerca de seis máquinas simples

When a young child decides to build a fort in the backyard, Grandpa comes forward to help. But they can't do it alone-they get help from the six simple machines,lever, pulley, inclined plane, wheel and axle, screw, and wedge. Told in cumulative rhyme, similar to The House That Jack Built, readers follow the building process to completion and discover the surprise reason it was built.