To celebrate Sept. 9, 2009, choose your favorite 4-digit number, whose digits are all different (like 2358).

Now scramble the digits to create a new number (like 5823).

Now subtract the smaller number from the larger.

Add the digits in your answer.

If the sum is a 1-digit number, then stop.

If the sum is a 2-digit number, then add those two digits.

You should now have the number 9.

Happy 9 day!

Art Benjamin is professor of mathematics at HMC. The explanation for this trick is given in his Discrete Math class (Math 55). It also appears in his book, "Secrets of Mental Math," and in his course “The Joy of Mathematics” produced by the Teaching Company (in Lecture 9, of course!).