Before 2004, game physics were largely cosmetic. You shot a barrel, it might change texture. In Half-Life 2 , the Source Engine introduced the Havok physics engine in a way that integrated directly into gameplay. The Gravity Gun wasn't just a weapon; it was a tool that turned the environment into an arsenal. Picking up a radiator to block a sniper round or using a sawblade to slice a zombie in half felt revolutionary, and frankly, it still feels satisfying today.