Generational puzzle game. You never quite forget the first time you play something like Portal. The ideas are insanely cool and you feel like a genius whenever you finally solve a level. I replay this game every year looking for another hit of that puzzle goodness, but nothing will ever replicate the first play. Valve did so much right with this game, and the only thing I wish it was longer. Thankfully there's an equally brilliant sequel that compounds on everything this game attempts to do. Short play and entirely worth your time.