Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Bc5/4. Bxf7/4...Kxf7/5. Nxe5/5...Nxe5

Nxe5
This is the obvious response. It seems that black has won 2 pieces, but white will regain one. He will then be a piece down for two pawns with no compensation, however. Since Black has many replies after 6.d4, all of which retain a winning position, the most popular move is 6.Qh5+.