The air pressure created by the clown's lungs on the inner walls of the balloon force it to expand. (Corbis Corporation)

A propeller converts through helical motion the energy supplied by a power source into thrust, a force that moves a vehicle forward in a fluid medium. They are used primarily for marine and aerial propulsion, but they are found on other technologies such as hovercraft and wind turbines as well. Propellers, which are essentially a series of twisted wings, or blades, connected to a central hub, are efficient energy transmission devices for those applications. The blades strike the air or water at a certain angle, called the pitch, and create an area of low pressure in front of the propeller. As a result, the blades generate thrust through either fluid or aerodynamic means by pushing forward through the low-pressure area. Slip, or the energy lost as the...