Get some people who are strangers to the boys to come along as passers-by in the street or road, and let the boys separately notice all about them; and after an interval ask each for a full description of each of the passers-by as to appearance, peculiar recognisable points, and what he guesses his business to be; or let each boy have two minutes' conversation with your friend and try to find out what he can about him in that time by questioning and observation.
Set a room or prepare a piece of ground with small signs, tracks, etc., read aloud the story of the crime up to that point and let each boy or each patrol in turn examine the scene for a given time and then privately give each his solution of it.
The very simplest, most elementary schemes should be given at first, and they can gradually be elaborated. For instance take a number of footmarks and spent matches by a tree showing where a man had difficulty in lighting his pipe, etc.
For a more finished theme take a mystery like that in "Memoirs of Sherlock Holmes" called "The Resident Patient." Set a room to represent the patient's room where he was found hanging, with footprints of muddy boots on the carpet, cigar ends bitten or cut in the fireplace, cigar ashes, screw-driver and screws, etc. Put down a strip or "stepping stones" of stuff, handkerchiefs, or paper on which the competitors shall walk (so as not to confuse existing tracks). Let each scout (or patrol) come in separately and have three minutes in which to investigate. Then to go out and give in his solution, written or verbal, half an hour later.
Let one patrol make tracks by carrying out such a series as that which D'Atagnan elucidated. The other patrol then acts as detectives and endeavours to unravel the mystery from the tracks and other sign.
PLAY.
Any one of Sherlock Holmes stories makes a good play.
BOOKS TO READ.
"Memoirs of Sherlock Holmes."
"Adventures of Sherlock Holmes."