Chess Opening Theory/1. e4/1...e5/2. f4/2...exf4/3. Nf3/3...g5/4. h4/4...g4/5. Ne5/5...Nf6

There's a pawn on g4 if White wants to capture it, but it isn't a free gift: Black is counter-attacking e4.
 * 6. d4 is the main line. White is attacking a pawn and Black is attacking a pawn, that's 1 pawn each, so rather than resolve either of those situations, White makes to 2 to 1 in her favour! This move also ensures that once Black kicks the knight away from e5 by playing ...d6, it will be able to retreat to d3 without blocking the d-pawn. Playing 6.d3 wouldn't have that advantage, and would also shut in the f1-bishop.
 * 6. Bc4 was the move favoured by the world champion of the pre-World Championship era, Paul Morphy. White capitalises on the e5-knight's attack on f7. Black will have to close the diagonal with 6...d5 7.exd5, after which material will be level but White's bishop will be blockaded by its own pawn.
 * 6. Nxg4 is also possible, but White is in danger of remaining a pawn down after 6...Nxe4 and will have to enter some complications involving a sacrifice of the rook on h1.