Chess Opening Theory/1. e4/1...c5/2. Nf3/2...e6/3. d4/3...cxd4/4. Nxd4/4...Nf6

= Sicilian 2...e6 = This is the flexible move. The Knight comes out to f6 and threatens the e4 pawn. It's important that Black determines how White deals with the e4 pawn before moving pawns further if White plays Nc3 then he can no longer play c4 first. 5.e5? is a common mistake as ...Qa5+ picks up material.

Theory table
1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 Nf6