To complete a well for production, it must first be inlaid with a steel pipe. First, a layer of cement is placed around the steel pipe, covering the well walls to inhibit penetration of undesirable fluids and the collapse of the walls. Then a special device is lowered down the shaft and activated from the surface, causing perforations in the steel and cement and opening small holes in the oil- or gas-bearing zones, allowing these fluids to flow into the well's interior. Next, another pipe with a smaller diameter (production column) is inserted into the well to carry the fluids to the surface. Finally, a set of valves, known as a christmas tree, is installed at the wellhead to control output.