Valve Corp. is giving away Portal, one of its best-known titles, to encourage use of the game to teach principals of physics and critical thinking. The only catch is that people must first install the company’s Steam game platform client to their PC or Mac. That’s free too, but it does require registration.

The cake may be a lie, but the free download is real. It’s part of Steam’s “Learn with Portal” campaign and runs until Sept. 20, when the game returns to costing $10. Although its interest in promoting education is genuine, undoubtedly Steam is hoping that the initiative will result in new customers for its online gaming service. Also, since Portal 2 came out in April, sales of the second game in the critically-acclaimed franchise could get a boost as well.

“One of the biggest challenges in teaching science, technology, engineering, and math is capturing the students’ imaginations long enough for them to see all of the possibilities that lie ahead,” Steam said in a statement. “Using interactive tools like the Portal series to draw them in makes physics, math, logic, spatial reasoning, probability, and problem-solving interesting, cool, and fun which gets us one step closer to our goal – engaged, thoughtful kids!”

Portal was originally released as part of a game bundle called The Orange Box. Players have to solve puzzles to escape the Enrichment Center for Aperture Laboratories, armed with a gun-like device that creates strategic teleportation connections between two flat planes (i.e. a wall or floor). The closest thing the game has to a villain is GLaDOS, the artificial intelligence in charge of Aperture Laboratories.

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