Gamespot extols the wonders of advanced physics in its preview of Half-life 2: "We were also given a demonstration of Half-Life 2's realistic physics, which will affect both the objects and the characters in the game. Objects will have realistic mass--in the pier sequence, one zombie hurled a barrel at Freeman, which rolled toward him but was stopped short by a shotgun blast. Objects have realistic buoyancy as well--some float, while others sink. Killed characters will slump down in lifeless heaps as "rag doll" physics kick in. If a zombie standing near a wall takes a close-range shotgun blast, you might see it slam backward against the surface and slowly slide down. The improved physics engine of Half-Life 2 should not only help make the game look more believable, but it will also figure into some of the game's puzzle-solving elements. As a conceptual demonstration of this, we watched as an oversized wrench was thrown into a piston-driven machine--and sure enough, the machine ground to a halt when the wrench got caught in the machinery." http://www.gamespot.com/pc/action/halflife2/preview_6026488.html