Originally Posted by
Chick-Dance
A teacher meets with 23 new students when they arrive. She tells them, "You may meet today and plan a strategy. But after today, you will have no communication with one another.
“One classroom contains two light switches labeled A and B, each of which can be in either the 'on' or the 'off' position. BOTH SWITCHES ARE IN THEIR OFF POSITIONS NOW. The switches are not connected to anything.
"After today, from time to time whenever I feel like it, I will select one student at random and escort her/him to that classroom. This student will select one of the two switches and reverse its position. He/she must move one, but only one of the switches. He/she can't move both but he/she can't move none either.
"No one else will enter the classroom until I lead the next student there, and he'll be instructed to do the same thing. I'm going to choose students at random. I may choose the same student three times in a row, or I may jump around and come back."
"But, given enough time, everyone will eventually visit that classroom as many times as everyone else. At any time any one of you may declare to me, 'We have all visited the classroom.'
"If it is true, then you will all get an A+ in this class. If it is false, and somebody has not yet visited the switch room, you will spend a full day with ACDriver."