“There are only 10 types of people in the world: those who understand binary and those who don’t.”
Figure it out yet?
“10” in binary numerals equals “2” in decimal numerals, so when you read the sentence, pronounce “10” as “two.”
In all fairness, a number should have a subscript when its base differs from ten. In this case, it should be written 102. But that wouldn’t make for much of a joke then, would it?