In binary, 10 is 2.
So, 2 makes sense since the joke talks about 2 kinds of people.
Since you didn't get it, you're obviously the type of person who doesn't know binary.
As I mentioned earlier, there's a shirt with that joke. You will only get 2 responses... those who laugh and those who don't. Don't who don't are the ones who don't know binary. Those who do are the ones who do know binary.
What is binary? It's a numerical system that consists of only 1s and 0s.
We use the decimal system that consists of 0, 1, 2, 3, 4, 5, 6, 7, 8, & 9.
If we were to count up with decimals, it would be 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15.
If we were to count up with binary, it would be 1 10 11 100 101 110 111 1000 1001 1010 1011 1100 1101 1110 1111.
