Another puzzle

Two prisoners are held in a dungeon. One night a mysterious visitor appears in their cell and offers them a chance to escape. It is only a chance because they must first reason to a decision which will determine whether or not they actually do go free.Their cell is at the bottom of a long flight of steps. At the top is the outer door. Three envelopes, marked X, Y and Z, are placed on
the table in the prisoners’ cell. One of them, they are told, contains the key to the outer door, but they may take only one envelope when they attempt to leave the cell. If they choose the wrong one, they will stay locked up forever, and the chance will not come again. It is an all-or-nothing decision.

There are six clues, A to F, to help them – or puzzle them, depending on how you look at it. Two are printed on each envelope. There is also a general instruction, on a separate card, which stipulates:

No more than one of the statements on each envelope is false.
On envelope X it says:
A The jailhouse key is solid brass.
B The jailhouse key is not in this envelope.

On envelope Y it says:
C The jailhouse key is not in this envelope either.
D The jailhouse key is in envelope Z.

On envelope Z it says:

E The jailhouse key is solid silver.
F The jailhouse key is not in envelope X.

The prisoners may look inside the envelopes if they wish, before deciding. They have five minutes to make up their minds.

Decide which envelope the prisoners should choose in order to escape from the cell.

The best way to do this activity is to discuss it with a partner, just as the two prisoners would do in the story. As well as deciding which envelope to choose, answer this further question:

Why is the envelope you have chosen the right one; and why can it not be either of the others?

Butterworth J. and G. Thwaites (2013) Thinking Skills. 2nd edition Cambridge: Cambridge

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