Two red and two green party hats are placed on four people and these people are arranged as shown - so that A can see B & C, B can see C, but C and D cannot see any of the others. (They all know that there are two red and two green hats.) The first person to correctly call out the colour of the hat on their head wins a prize. Who wins, given that nobody simply guesses?
Since A is silent, A must see a red and a green hat. B can see C, so whatever colour hat C has on, B knows to say the other colour.