Now you're thinking about portals.

I've started playing Portal. It's a ridiculously fun platform puzzler that's really well designed (some jumping is tough for me to execute, but failure doesn't usually set me back very far). And there's all sorts of neat little touches (the cake is a lie!) to lend atmosphere to what's pretty much a series of boxy puzzle rooms .

Shamus Young has some speculations on portal physics, building off of his review of the game. Really, this is like the various conundrums about time travel; when you break the laws of physics in one way, there's no reason to expect the rest of it to accommodate that breach. But it's still fun to think about. (I've been wondering why I didn't slice my character in half when I tried walking into the portal's edge, aside from the fact that doing so would make the game fiendishly difficult, because every just-missed jump would result in the character's death.)