Your goal in Portal is to look for non-metallic surfaces, comprehend how they're oriented in relation to each other, then place portals in the appropriate positions to allow your character to reach the exit. In the tutorial stages the rooms we moved through were nearly entirely metal, meaning the puzzles weren't all that difficult since there were limited spots to plant portals. The first area required we toss a portal up on the ceiling to allow an oscillating energy sphere to pass through and exit down to a generator that powered a lift and allowed us to exit. The next few were a series of momentum puzzles, solved by dropping a portal onto a floor section and jumping through from a great height. Since we'd put a second portal on a vertically oriented tile high above, our ensuing exit tossed us onto a higher platform, allowing us to reach the door and head to the next section. In the final area, we had to do a series of platform ascension exercises using the same method, then deal with a box at the top level. We grabbed the box and dropped down to place it on a pressure pad on a lower ledge. By portal hopping back up to the top ledge, we saw the door was now open, and we proceeded through for the end of the demo.

The game will consist of 19 levels that will amount to about four hours of gameplay for an average player, with the most complex levels purportedly taking about half an hour to figure out. Of course, since Portal is only one piece of a considerably value-packed box of games, you can't complain if it seems a little short. Valve hasn't committed to providing any further levels for Portal via download after release, but they may not have to. The PC version of the game will see a level editor not long after it becomes available at retail, so we imagine there will be no shortage of user-created levels to keep you portal hopping for quite a while. (Valve hasn't determined how, or even if, these levels will be available to the console versions of Portal, however.)