Coin Flipping Game

Player VS Player Player VS Computer

Prisoner A, please chooses a coin to flip.

[Player VS Player Mode]

Nim Sum Calculator: The nim sum of

is 13.

Calculate Automatically input the index of black coins

Start Again

Restart

There is an 8 x 8 chessboard in a room and each square is occupied by a coin with random face up or tail up.

Prisoner A enters the room and sees the Warden pointing to one of the squares (indicated by a red arrow).
Prisoner A chooses any one of the 64 coins (the same or different than what Warden has chosen) and flips the coin.

Then Prisoner A leaves the room without any communication with Prisoner B.
Now Prisoner B enters the room and guesses the square that the Warden has pointed to by looking at the chessboard.

How did the Prisoners make it possible?