and 11/11/11.

Take the last 2 digits of the year in which you were born.

Then add it to the age that you will be in 2011.

What do you get?

Yes, the sum will always equal 111 (if you were born in the Twentieth Century).

So, why does this work?

Let x = last 2 digits of the year in which you were born.

If you were born in the Twentieth Century,

Then your age this year will be 11 + (100 - x).

The 11 represents the number of years from 2000 to 2011.

The quantity (100 - x) is your age as of the year 2000.

So, when you add the two quantities together, you obtain:

x + (11 + (100 - x)), which equals 111.