[<< wikibooks] Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. Nc3/4...fxe4/5. Nxe4/5...d5/6. Nxe5/6...dxe4/7. Nxc6/7...Qd5/8. c4/8...Qd6/9. Nxa7/9...Bd7/10. Bxd7/10...Qxd7/11. Qh5/11...g6/12. Qe5/12...Kf7/13. Nb5
Black's c7 and e4 pawns are threatened. 13...c6 saves one and, by kicking the knight, gains the necessary tempo to save the other.
Black still isn't worried about Qxh8 which can be met by ...Nf6, ...Re8 and ...Bc5 winning the queen for two rooks.