There are 10 kinds of people in the world...

An observation found on a t-shirt my son owns, and which I also noticed today on Hacker News:

There are 10 people in this world, those who understand binary and those who don't.

 The first reply on Hacker News:

There are 10 kinds of people in this world: Those who understand ternary, those who don't, and those who mistake it for the binary system.