Working on the car builds Mechanical skill, though only half as quickly as studying, and also builds enthusiasm for the Tinkering hobby. A Sim can work on the body or on the engine, and one Sim can work on the body while another works on the engine. If the car is not being worked on, a Sim can rev the engine. This does not appear to be required, but adds a touch of realism. Fully restoring either the body or engine takes about six game hours of labor, but Sims will probably need to take breaks, as working on the car is hard on the comfort, energy, and hygiene motives. While a Sim is working on the engine, the engine may backfire and belch black smoke, which will cause the Sim to stop working on it.