Vegetable fats and oils are what are most commonly called vegetable oils. These are triglyceride-based, and include cooking oils like canola oil, solid oils like cocoa butter, oils used in paint like linseed oil and oils used for industrial purposes. Pressed vegetable oils are extracted from the plant containing the oil (usually the seed), using one of two types of oil press. The most common is the "screw press", which consists of a large-diameter metal screw inside a metal housing. Oil seeds are fed into the housing, where the screws mash the seeds, and create pressure which forces the oil out through small holes in the side of the press. The remaining solids, called "seed cake", are either discarded or used for other purposes. Oil presses can be either manual or powered. The second type of oil press is the "ram press", where a piston is driven into a cylinder, crushing the seeds and forcing out the oil. Ram presses are generally more efficient than screw presses.