To Introduce the Idea of Paradox
from Martin Gardner's New Mathematical Diversions
From Scientific American
(1) On a piece of paper write the phrase,
Sometime before the end of this class period, you will write
the word "NO" on a note card which I have given you.
(2) Fold the paper and put it in an envelope. Give this to a student.
(3) Now give that student a blank note card and explain that you have just
written a prediction about an event which will definitely happen or
not happen during this period.
(4) Request that he write "YES" on the card if he guesses it will happen
or "NO" on the card if he guesses it will not happen.
(5) Ask him to fold the card and give it to another student for safekeeping.
(6) Now ham it up - 50-50 chance; I'll give 10 to 1 odds that he is wrong.
Offer NO HOMEWORK if he is right.
(7) Then get the response card from the student and read the prediction aloud.