A car jack is a mechanical device that allows drivers and mechanics to get underneath a car, usually to change a tire, oil or some body part like brakes or struts. There are two kinds of car jacks: hydraulic and screw types. Most car jacks that are included with cars are screw types. Of the screw-type mechanisms, there are scissor jacks, common in newer cars, and bumper jacks, common in older cars
The jack is usually found in the trunk of the car with the spare tire. Both the scissor jack and the bumper jack use an arm to allow the car owner to lift the car. If changing a tire, be sure to loosen the lug nuts before you raise the car. The scissor jack is designed to turn the screw with the arm in a clockwise motion to raise the platform that the car rests on (or notch that fits into a hard point on the car's undercarriage). As you turn the handle, a large screw pushes the platform up by shortening the distance between end points; the mechanism crawls along the grooves of the screw so as to lift the car with relatively little effort.
There is a lot of weight supported by the jack. Make sure to follow all the safety suggestions in your car manual, and that the jack is placed on a flat, concrete surface and no one is sitting in the car. The car descends by reversing the process: turn the screw counterclockwise, making sure to tighten the lug nuts after the car's tire has minimal weight on the street's surface, and then lowering the jack the rest of the way.
