Valves are commonly used in game worlds as an element characters may interact with to change or alter their environment. They may be positioned on pipes, on doors, or elsewhere. Turning them may flood an area, empty a pool, change the flow of water or gas through a pipe system, or have other effects. They may be used as a terrain puzzle, where the proper combination of valves must be turned. In narratives, valves may commonly become stuck at inopportune times.