When Valve's Steam digital game distribution service , the company made Portal free for a limited time to entice Mac gamers; Friday's promotion, however, has been touted by Valve as a learning initiative, helping kids in classrooms learn about physics through gameplay.

As those who have played the game might know, , which premiered in 2007 as part of Valve's The Orange Box to great success, focuses on a series of physics puzzles involving momentum redirection and object manipulation. A sequel--aptly named --was released earlier this year.

Portal normally sells for $10 in Valve's store; if you haven't played it, now's the time to strap on a set of and start testing. You can download it for free using the store (though you'll need to create a free account if you don't already have one).