There is a key hidden in one of three boxes and each box has a coin on top of it. Can you use the coins to let your friend know where the key is hiding?

You and your best friend are locked in jail for no reason at all, but you are given the opportunity to escape. You are taken to a room that has three opaque boxes. The key to your cell will be put inside one of the boxes, and then a (regular) coin is placed on top of each box. You may pick a single coin and reverse its face up, and then your friend will enter the room.

When your friend enters the room you are not allowed to talk, and your friend must open a box. If your friend opens the box with the key, you are set free. Otherwise, you are locked for eternity...

What is the strategy that you and your friend should agree upon, so that your friend can always find the key?

Give it some thought...

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The solution to this problem will be posted [here][sol] after this problem has been live for 2 weeks.
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[sol]: ../../solutions/hidden-key