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A student is dressed to look like a murderer (!) and stands on the carpet piece marked ‘Hall’.

Another student (acting as Sherlock Holmes) explains that the murderer will be tracked down in the house and arrested. Sherlock is blindfolded.

A second student throws the dice, and states the number on the dice out loud. The murderer walks through that many doors on the plan of the house.

Sherlock says, “I know the murderer is not in the kitchen (or name of other room), so will someone please take away the kitchen.”

A third student takes away the piece of carpet labeled ‘kitchen’. The murderer from now on cannot step on the area which was once the kitchen.

The process is repeated a number of times until Sherlock can say, “I know that the murderer is in the ……….. (name of room), you are under arrest.
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How does the trick work?
Well, if the murderer starts in the hall, four rooms are one door away (study, bathroom, dining room and conservatory). We shall call these rooms as odd rooms.

The other rooms are two doors away. We’ll call these rooms as even rooms.

If the number on the dice is even, Sherlock knows that the murderer must have walked into an even room so can ask for one of the odd rooms to be  removed.

The process continues. Sherlock uses the principles that:

Odd + odd = even

Even + even = even

Even + odd = odd

Odd + even = odd

… to keep track of the rooms the murderer can’t be in. By process of elimination the correct room can be found (In practice Sherlock must reserve the right not to remove a room in certain cases)

This could be followed by whole group discussion about how the trick was performe (using student’s own theories and words.
Students in pairs try to perform the trick, using the diagram and a counter to represent the murderer.
Acting as Sherlock requires a great mental effort keeping track of the rooms.

A large house plan made from nine pieces of carpet (each 3m by 3m) is set out in the middle of a large room as shown:
A large house plan made from nine pieces of carpet (each 3m by 3m) is set out in the middle of a large room as shown:

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