Portal is one of the rare games that can change the way we think about the world, how we view the physical space around us. Valve is now offering the first Portal title for free, provided that you download a copy of the program for either Mac or PC before September 20. This is the full version of the game, and it will remain in your library after that date; you simply must download the title before next Tuesday.

"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 company wrote on the Learn with Portals site. "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!"

This isn't idle talk; Valve has invited schoolchildren into the company's offices to help them create levels for Portal. This instruction goes deeper than just allowing kids to fool around with a video game, as the children learned about spatial reasoning, engineering, and even mathematics. It's an important lesson: what might seem like a dry subject in the classroom has amazing applications outside of it. It's one thing to try to explain to a child how air flows over moving objects, and it's quite another to twist the fins on the body of a model rocket to show them how to create torque.

This is a great way to spread the love of science and gaming to children, and if you haven't played Portal until this point, you now have no excuse.