The classic answer to the riddle posed to my students in Day 9 – the riddle where there are two doors (one with great wealth the other with painful death) and two guards (one always tells the truth the other only lies).
What would be the one question you could ask – and you can only ask to get the riches (and not die – that is very important)?
You ask (either one) what door will the other guy say is the door to riches?
Both guards will tell you the incorrect door.
If you ask the truthful guy he will truthfully tell you that the liar would tell you the door that doesn’t lead to treasure (if you asked him directly)
I you ask the liar, as he knows that the truthful guy would tell him the right door but he has to lie, he would tell you that he would say the incorrect door.
Working on helping my learners see math as so much more than just numerals added subtracted multiplied and divided. Hmmm that might be my tag line on my electronic signature before too long!