Stamps - Alice Becky Claire
Alice, Becky and Claire are perfectly logical.
Each can make perfect deductions - and each of them knows that the others can too.
(in other words, Becky is analyzing and using the given info from Alice, and Claire is analyzing and using the given info from Becky and Alice)
The three of them are shown seven stamps:
two red stamps, two green stamps and three yellow stamps
They are then blindfolded and one stamp is stuck on the forehead of each girl: the remaining four are hidden away.
The blindfolds are removed and Alice is asked: “Do you know anything - positive or negative - about the color of the stamp on your own forehead?” Alice replies, “No.”
Becky is asked the same question and she also replies, “No”
Claire immediately breaks into a smile - because she knows the color of the stamp on her forehead. What color must it be and why?