1. c2c4  1... g8f6  2. b1c3  2... e7e5  3. g1f3  3... b8c6  4. g2g3  4... f8b4  5. f1g2  5... e8g8  6. c3d5  6... e5e4 This is considered the best reply according to theory, although other moves are possible.