E4 D5

If he plays Qxd5 you attack his queen with your knight and you gain advantage because he has to move his queen. Instead, if he plays Nf6, you mustn't play c3 (for maintaining the pawn) because he sacrifices a pawn with c6 but he gains two knights developed after dxc6 Nxc6; you have to let him recapture the pawn and the games continue.
This opening is called Scandinavian.