For several years, Valve secretly worked on "Half-Life 2". Valve developed a new game engine, Source, which handles the game's visual, audio, and artificial intelligence elements. The Source engine comes packaged with a heavily modified version of the Havok physics engine that allows for an extra dimension of interactivity in both single-player and online environments. In the episodic games that followed "Half-Life 2", Valve made minor tweaks to the game's engine. In "Episode One", Valve modified Alyx's AI to allow her to react to player actions. The game runs on an upgraded version of Valve's proprietary Source engine, and features both the engine's advanced lighting effects, and a new version of its facial animation/expression technology.