Garry's Mod is a physics-based sandbox game built on Valve's Source engine. In its base mode, there are no objectives. Players can spawn non-player characters, ragdolls, and props, then manipulate them using a physics gun that can pick up, rotate, and freeze objects in place. A tool gun provides additional functionality such as welding objects together or adjusting facial expressions on ragdolls. The game supports extensive Lua-based modding, and community-created game modes like Trouble in Terrorist Town and Prop Hunt have become widely popular in their own right. Originally released as a free mod for Half-Life 2 in 2004, it became a standalone commercial release in 2006 and holds the record as the best-selling PC-exclusive game.