Thirteen is the number that comes after twelve and before fourteen. It's written as 13 in numerals, and it's the age when someone becomes a teenager (along with fourteen, fifteen, sixteen, seventeen, eighteen, and nineteen, which all end in “teen”).

In mathematics, thirteen is a prime number, meaning it can only be divided evenly by itself and 1. It's also considered an odd number because you can't split thirteen objects into two equal groups.

Many cultures consider thirteen unlucky, which is why some buildings skip labeling the thirteenth floor (jumping from 12 to 14 instead) and why some people feel nervous about Friday the 13th. But there's no scientific reason for this superstition. In fact, some people consider thirteen lucky. The United States began with thirteen colonies, which is why the American flag originally had thirteen stripes and thirteen stars.

In a standard deck of playing cards, each suit contains thirteen cards. A baker's dozen means thirteen items instead of twelve.