What do sailboats and theater curtains have in common? Your readers will learn the answer; it's pulleys. The pulley plays an important role in modern daily life. This simple machine makes raising and lowering heavy loads much easier. From fixed pulleys to moveable and compound pulley systems, readers gain an in-depth understanding of the forces that make these machines work. Readers learn the STEM concepts behind how pulleys have vital roles in compound machines, from allowing cranes to work on buildings to giving bicycles their smooth ride. Eye-catching photographs including a picture gallery of common objects that use pulleys give readers an in-depth understanding of these revolutionary devices.