A control device is a tool used to regulate or direct the behavior of a system. A servo, which is a five-letter word, is a type of motor that is used to control the position of a mechanism. RADAR, also a five-letter word, is an object-detection system that uses radio waves to determine the range, angle, and velocity of objects. A REIN, a four-letter word, is a strap attached to a horse's bridle that is used to control the animal's movements. Lastly, a KNOB, another four-letter word, is a circular handle used to adjust the settings of a machine.