Mechanical engineering is the second largest engineering discipline and one of the oldest. Mechanical engineers apply the principles of mechanics and energy to the design of machines and devices. Mechanical engineers are involved with the design of structures, operation and maintenance of mechanical systems. Mechanical engineers design automobiles, trucks, airplanes, and trains for transportation; tractors for food production; interplanetary space vehicles; copying machines, fax machines, staplers, and mechanical pencils used in the office; and lathes, milling machines, grinders, and drill presses used in the manufacture of goods. Mechanical engineers also design and operate power plants and are concerned with the economical combustion of fuels, the conversion of heat energy into mechanical energy, and the use of the mechanical energy to perform useful work.