I have 3 identical boxes with a lid on each and a $100 note in one of the boxes.
You choose a box that you think contains the $100 note.
What is the probability of you choosing the one that contains the $100 note?
After you make your choice, I open one of the remaining 2 boxes ..... an empty box (as I know which box has the $100 note, I choose an empty one , note: they may both be empty if you have chosen correctly) and show it to you.
Now what is the probability of you having chosen the box with the $100 note?
After I reveal an empty box, I offer you the option of switching your original choice to the only other un-opened box.
