A grease gun is a tool used to inject lubricant into difficult areas, usually through an access point called a grease nipple. Grease guns may be hand-powered or pneumatic. In one type of hand-powered grease gun, the grease is forced out by hand cranking the trigger mechanism of the gun. This applies pressure to a spring mechanism behind the lubricant, which expels the grease. In the other hand-powered variety, in which there is no trigger mechanism, the grease is forced out by the back-pressure built up by pushing on the butt of the gun. This slides a piston through the body of the tool, pumping grease out of the aperture. In the pneumatic type, compressed air is directed to the gun by hoses and serves to force the grease through the aperture. The grease gun can be loaded with any of various kinds of lubricants, but usually a thicker, heavier type of grease is used.