111

If you were born before the year 2000: add the last two digits of the year of your birth and the age you will be on this year’s birthday. The answer will be 111. If you were born in 2000 or later, the answer will be 11. If you were born in 1899 or earlier, the answer will be 211. (If you were born in 1899 or earlier, you’re probably not reading this post. If you were born in 2000 or later, you should be out playing.)

The year 2011 is the year of 1/1/11, 1/11/11, 11/1/11, and 11/11/11. And adding age to year always ends in eleven. Pretty mysterious, eh? Not really. Snopes has an explanation. Maths Questions has a more elaborate one.

[It amuses me that I learned this trick not from an online source but from my dad, who got it from a neighbor. Neighbors: the original Internets.]