There were 4 angels on a Christmas tree (besides other frou-frou). Two had a blue aureole and two
yellow, however none of them can see behind his head. Angel A is on the highest place and he can
see angels B and C, which hang below him. Below hangs angel B, that can see only angel C under him. Angel C can't see anybody, because angel D hangs under a twig (nobody can see him and he can not see anyone either).
Which one of them will be the first to guess, what his own aureole is?
There are 2 possible solutions:
1. if angels B and C had aureole of the same color, then angel A must have immediately
said his own color (other then theirs),
2. if angels B and C had different colors, then angel A must have been silent and that
would have been a signal for angel B, who could know (looking at angel C) what his
own color is (the other one then C had).