From IGN:

"But seriously, shizzle aside, the Portal demo blew our minds. In the demonstration, Newell walked us through the concept. I'll back up a step. If you've played or heard of Humanhead's Prey, then you know the game offers organic portals throughout a giant sentient alien ship. The portals are dimensional doors into different areas in the ship, and they're graphically and conceptually well implemented and work seamlessly. They're also stationery. In Portal, a seemingly independent game that ships with the PC version of Half-Life Episode 2, and the bundle of Half-Life 2, Episode 1, 2, and Team Fortress 2 on Xbox 360 and PS3, players use a tool that creates portals on the fly. Yes -- a gun that creates instant openings through walls that transport you in the bat of an eye to another place."

The folks over at IGN have ruffled up their new preview of Half-Life 2's next expansion pack, Episode Two. Here's a divvy from it:

"On top of Source's new cinematic physics feature, the engine has also been updated with soft particles and a few other minor tweaks. Gabe Newell did mention that Episode Two will feature not only new environments (one of which being giant outdoor spaces) and creatures (ala the tripods), but new weapons as well."

For further embellishing, check out the unabridged preview, as per usual.