What is the shortest number of moves from the starting position required for Black to mate White with a pawn? Interesting puzzle that is quite hard to stop.
4 moves, as far as I can tell.

[FEN ""]
1. e3 e5 2. Ke2 Qh4 3. Kf3 d6 4. Qe2 e4#


Another one:

[FEN ""]
1. d3 d5 2. Kd2 e5 3. Kc3 Be6 4. Qd2 d4#


And one more:

[FEN ""]
1. e3 e5 2. Ke2 d5 3. Kd3 Qf6 4. Qe2 e4#


Same theme for all of them, really.