Three men apply for a vacant post. The prospective employer, liking each
applicant equally well, proposes a test.
The light will be extinguished and the employer will mark a cross in chalk
on the forehead of each man. He has chalk of two colors, blue and white,
and the cross may be either one of the two.
The light will then be turned up and any man who can see one or more
blue crosses is to stand up.
THEY ALL STAND UP.
The employer then asks if any man can state the color of the cross on his
own forehead. After an interval, one man states that the cross on his head
is blue.
HOW DID HE KNOW ?
The fact that all the men stood up shows that there must have been either
two or three blue crosses. (If there had been only one blue cross, the man
bearing it would not have been able to see a blue cross and therefore
would not have stood up.)
If any of the three men could have seen a white cross he would at once
know that the other two must be blue, because there must be at least two
blue blue crosses for all the men to have stood up.
The fact that there is an interval before anybody speaks shows that nobody
has seen a white cross. Therefore the man knows that the cross on his own
forehead must be blue.
(Editor’s note : The moral of this brain teaser is – Read Annemann’s
Complete One Man Mental and Psychic Routine and have a convex
mirror reflector.)