Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. d3/4...fxe4/5. dxe4/5...Nf6

There are a few minor moves here, but if White notices that she doesn't have to directly defend the e-pawn, then 6. O-O makes the most sense. 6...Nxe4? in response would be a tactical error.