I call it "Magic Numbers," because it makes me look really smart. Almost "magically" smart. My friend does this with her first graders, telling them that if they come on the last day of school she will share the "secret" of how it works.
Luckily for you, you won't have to wait until the last day of school to find out how it works.
Wait - maybe I could string you along for awhile and not share the secret for a bit.
Have your students choose any number between 1 and 31. DON'T LET THEM TELL YOU THE NUMBER, but they need to either write it down or share it with a buddy.
Show your students a card and ask, "Is your number on this card?" If the answer is yes, look at the number in the top left hand corner. If the answer is no, don't do anything.
Show the next card. This time if the student answers yes, add it to the number from before. Keep a running total. By the time you work through all of the cards and have added all of the numbers up you will have the students number.
It really is magic. What a great way to fill in a small bit of weird time when you are waiting for something.