Inspired first-person puzzle game Portal is free to download on Steam until 20th September.
Maker Valve is running the deal to showcase how Portal and Portal 2 can and have helped kids grasp trickier aspects of science in an enjoyable way.
Apparently Portal makes things like physics and problem solving "cool and fun". And that "gets us one step closer to our goal: engaged, thoughtful kids!"
Portal, a short game, is a calm and bullet-free puzzle solving experience. Twists of humour and taxing, portal-based conundrums made it one of the best games of 2007.
Eurogamer's Portal review awarded 9/10.
Video: Portal finished in nine minutes. It's not always that short.