There are 10 kinds of people in the world:

Those that understand binary, those that don't, and those who weren't expecting a ternary joke.

Pro tip: if you want to get someone a birthday card, just get them a card that says "10 years old!" on the front. Then use a sharpie to put an asterisk after the "10" and then on the inside of the card put: "* (in base X)", where X is how many years they are turning. Worked well for my 2 year old daughter as well as my 57 year old dad.

There are 10 kinds of people in the world:

Those who understand binary, those who don't, and those who understand that "10" can represent any number if you change the base.

(Changing joke this to work amusingly for irrational bases like the golden ratio base[1] is something I haven't yet figured out.)

[1] https://en.wikipedia.org/wiki/Golden_ratio_base