A gravity gun is a concept in video games, particularly first-person shooters using an advanced physics engine, whereby players can directly manipulate objects in the world, often allowing them to be used as projectiles against hostile characters. A gravity gun is usually manifested as an in-game weapon, available from the player's inventory or weapon selection. The concept was first popularized by the gravity gun found in Valve Software's Half-Life 2; however, a similar concept was used by developers id Software during the production of the earlier game Doom 3, eventually leading to the introduction of a physics-based weapon in the expansion pack Resurrection of Evil. Later games have been influenced by the success of these physics-based weapons, adopting their own styles of comparable abilities or weapons.