There are 10 kinds of people in this world.

• Those that know binary.
• Those that don't.
• And those that didn't expect this joke to be in base 3.

EDIT:

A statistician, a biologist, and a mathematician are sitting in a cafe watching the building across the street. 2 people enter the building. Some time later, 3 people come back out.

• The statistician says, "Our initial count must've been off."
• The biologist replies, "I think they probably reproduced."
• The mathematician observes, "Now if exactly one person enters the building it will be empty."