Before performing this trick, prepare a set of four cards on cardstock.
Here is what the cards should like like:
Yes, notice that this is a "marked deck" of cards.
I cut out tiny corners on the first three cards --
    one corner off the January card,
    two corners off the February card, and
    four corners off the April card.


Now place the cards face up on a table and ask a friend to secretly pick a month.

Turn your back and have your friend select the cards that have her month on it.
This could be 1, 2, or 3 cards, depending upon the month selected.

Have your friend turn those cards containing her month face down.

Have your friend remove any of the cards that do not contain her month on it.

Then you turn around and count the corners of the cards that are cut off.
On the card that has no corners cut off, add 8 to the total.

Your total of "corners cut off" will be the number of the month...
1 is January, 2 is February, ... 12 is December.



Why does this work?

This trick uses Base 2 arithmetic (There is a similar Base 2 trick under the Number Tricks).
Represent each month's number (1 to 12) in base 2.
For example, January is the first month, and 1 in base ten equals 1 in base two (or 0001 in the table below).
November is the eleventh month, and 11 in base ten equals 1011 in base two.
Listed below are all the months and their base 2 representation.
Note that the "August" card has all the months containing an 8: August through December.

Month Picked August Card
8 		April Card
4 		February Card
2 		January Card
1
(January)   1 =   0 0 0 1
  (February)   2 =   0 0 1 0
(March)   3 =   0 0 1 1
(April)   4 =   0 1 0 0
  (May)   5 =   0 1 0 1
(June)   6 =   0 1 1 0
(July)   7 =   0 1 1 1
  (August)   8 =   1 0 0 0
(September)   9 =   1 0 0 1
(October)   10 =   1 0 1 0
  (November)   11 =   1 0 1 1
(December)   12 =   1 1 0 0