Originally posted by English Tal
pppppppp (lower case p's)
PPPPPPPP (Upper case p's)
The Uppercase army has a different parity than lowercase army (one has made an even number of moves, and the other an odd number of moves, depending on how the board is rotated). A difference in parity happens only after White has moved. Since a lowercase 'n' is giving check to the enemy K, the lowercase pieces must be White.
1.Nf3 Nc6 2.Ne5 Nd4 3.Nc6 Nf6 4.Nxd8 Nd5 5.Nc6 Ne3 6.Nc3 Nxd1 7.Nd5
Nc3 8.Na5 Ncb5 9.Nc4 Nf3+ 10.Kd1 Rg8 11.Nf6+
Edit: I knew this looked familiar. R. Smullyan, Chess Mysteries of Sherlock Holmes
, p. 87.