Explain this joke
 

"A biologist, a physicist, and a mathematician were sitting in a street café watching the crowd. Across the street they saw a man and a woman entering a building. Ten minutes later they reappeared together with a third person. “They have multiplied,” said the biologist. “Oh no, an error in measurement,” the physicist sighed. “If exactly one person enters the building now, it will be empty again,” the mathematician concluded."

I don't get it.

Are you wondering about the math bit? It's because you started with 0 (empty), you added 2, subtracted 3, and need to add another one to get it back to 0.

I get it xD

i... barely get it... xD but it doesnt seem funny since i had to strain my pathetic brain to understand it...

Eh, I don't think it is really meant to be funny, in the traditional sense. I think it's meant to be one of those "Ha! I understand it, therefore I am superior!" type jokes. The kind Mensa members like to tell each other. Humorless bastards.

I am superior. lulz

But, how do you not know that there was someone already in the building; Going from 1 + the 2 that entered = 3 then - 3 would equal 0, then another 1 would make 1...
The joke didnt say the building was empty.

Where true genius lies.

Because a lot of mathematicians like to assume things.

I didn't write the joke. Don't shoot the messenger, heh.