To be a mechanical engineer, you need to be a bit of a know-it-all. Mechanical engineering is one of the largest and broadest engineering disciplines, encompassing the development of all types of mechanical systems. Mechanical engineers are experts in thermodynamics, heat transfer, structures and materials, fluid dynamics and advanced control systems. They use their knowledge to improve many aspects of our lives. In health care, they contribute to new surgical tools, imaging systems and prosthetic devices. In security, they design sensors that detect chemical threats and monitor the structural soundness of buildings and bridges. Mechanical engineers also help design and manufacture the computers, cell phones, microwave ovens, refrigerators, air conditioners and other devices that improve the quality of daily life.