Carlsen decided to sacrifice a  pawn.
1. e2e4  1... e7e5  2. g1f3  2... b8c6  3. f1c4  3... f8c5  4. c2c3  4... g8f6  5. d2d4 The good old classical  line that So already used to beat the world champion earlier this year. 5... e5d4  6. e4e5  6... d7d5  7. c4b5  7... f6e4  8. c3d4  8... c5b6  9. b1c3  9... e8g8  10. c1e3  10... c8g4 The  pawn structure now is very much like the one in the Yates variation of the Ruy Lopez. White has a strong center, but Black's free piece play allows  him a chance to eventually attack and destroy the center. 11. h2h3  11... g4h5  12. a1c1 So deviates from the move that brought him success earlier, and  chooses a promising side line which he obviously analyzed extensively as  well. 12... b6a5 Carlsen bets on the \"outpost play.\" In the meantime,  he frees himself from the passive bishop. 13. e1g1  13... a5c3  14. b2c3  14... c6a5 That was the idea. The knight is about to step  on the c4-square. In hindsight, this seems slow though. 15. g2g4 So's plan is clear: he needs to bring his huge kingside mass  into motion. 15... h5g6  16. f3e1 The f-pawn is unleashed. 16... f7f6  17. f2f3  17... c7c6  18. b5d3  18... e4g5  19. d3g6 A very strong move. The tripled pawns  will be huge targets. 19... h7g6  20. e3g5  20... f6g5  21. d1d3 