Compactor is a machine or mechanism used to reduce the size of waste material through compaction. Normally powered by hydraulics, compactors take many shapes and sizes. In landfill sites for example, a large bulldozer with spiked wheels is used to drive over waste deposited by waste collection vehicles. Waste collection vehicles themselves incorporate a compacting mechanism, which is used to increase the payload of the vehicle and reduce the number of times it has to empty. This usually takes the form of hydraulically-powered sliding plates which sweep out the collection hopper and compress the material into what has already been loaded. Different compactors are used in scrap metal processing, the most familiar being the car crusher. Such devices can either be of the pancake type, where a scrap automobile is flattened by a huge descending hydraulically powered plate. The other type is the baling press, where the automobile is compressed from several directions until it resembles a large cube.