Answer: Birthplace

Social Security numbers, the ubiquitous I.D. number assigned to nearly every American citizen at birth or upon immigration, have a little Easter egg of sorts hiding in plain sight. It’s a small detail—and one you’d likely never notice unless you happened to both have an eye for number patterns and a job that exposed you to lots of Social Security numbers. Social Security numbers were, for decades, linked to the geographic region they were issued in.

How can you pull this information out of a Social Security number? The first 3 digits of the number are known as the “Area Number”, much like a phone number’s area code, and throughout the 20th century and into the early 21st, they were linked to specific regions (and even, within their blocks, specific issuing offices). If your number starts with 268-302, you were born in Ohio. 486-500? Missouri. 575 or 576? Aloha, you are Hawaiian. 525? You’re from a land where the air is dry and the UFO sightings are numerous, New Mexico.

You can also, albeit less reliably, estimate a person’s age based on their number, as numbers were typically issued in sequence across a given region. Armed with that knowledge, it’s a pretty safe bet that a person with a Social Security number that starts with 268, to use our previous example of Ohio-area numbers, is older than someone with a number that starts with 300 (a later number in the Ohio block).

Alas, if you were planning on using this newfound knowledge as a parlor trick, however, not so fast. As of June 25, 2011, the Social Security Administration now assigns the numbers randomly, and not based on the geographic location they are issued in, which means with each passing year, the number pool is more randomized.