Valve has opened up a window of time during which the original Portal can be downloaded free of charge. The publisher has launched a Learn With Portals website encouraging teachers to use its physics-based puzzle game to advocate the game's implementation in educational programs. As part of that initiative, it is allowing Portal to be downloaded free of charge through September 20.

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"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," the site explains. "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!"

Originally released in 2007 alone or bundled with other Valve games in The Orange Box, Portal dropped players in the role of a human test subject forced by a homicidal AI named GLaDOS to solve spatial puzzles with the aid of an experimental "portal gun." The gun let players create circular doorways on flat surfaces that connect to one another, no matter how far apart they may be. While it begins as a simple series of lab tests, Portal eventually sees players attempting to escape their imprisonment with a little thinking outside the box and judicious abuse of the laws of physics.

Portal ordinarily sells on Steam for $20, but it is currently available through Valve's Steam digital distribution storefront free of charge. For more on the game, check out GameSpot's review.