Portal

Portal grew out of a student project, Narbacular Drop: its authors were hired by Valve and allowed to develop the idea on the Source engine. There is one rule. The gun places two openings on suitable surfaces, a body passes from one into the other, and its speed is preserved on the way through. All the puzzles rest on this conservation of momentum: a fall from a height turns into a run-up, and the run-up into a jump across a chasm.

The campaign is short and built as a sequence of lessons without a single tutorial caption. Each room introduces one new property and makes you apply it before the exit opens; the next room builds on the previous one and adds one more. The voice of the laboratory comments on what's happening and gradually stops hiding how the tests end, so the story shows through the procedural announcements. The game came out as part of The Orange Box bundle alongside Half-Life 2: Episode Two and Team Fortress 2, that is, as an add-on to a big release, and after that it became a reference point in its own right for first-person puzzle games.