Rowing, often referred to as crew in the United States, is a sport with origins back to Ancient Egyptian times. It is based on propelling a boat (racing shell) on water using oars. By pushing against the water with an oar, a force is generated to move the boat. The sport can be either recreational, where the focus is on learning the technique of rowing, or competitive, where athletes race against each other in boats. There are a number of different boat classes in which athletes compete, ranging from an individual shell (called a single scull) to an eight-person shell with coxswain (called a coxed eight).