Most of the time, large powerful fans blow air through the tube. The object being tested is held securely inside the tunnel so that it remains stationary and does not move. The object can be an aerodynamic test object such as a cylinder or an airfoil, an individual component, a small model of the vehicle, or a full-sized vehicle. The air moving around the stationary object shows what would happen if the object was moving through the air. The motion of the air can be studied in different ways; smoke or dye can be placed in the air and can be seen as it moves around the object. Coloured threads can also be attached to the object to show how the air moves around it. Special instruments can often be used to measure the force of the air exerted against the object.