How do zombies know that you aren't another zombie when they attack?

Question

Zombies never attack other zombies. How do they know that you are alive and not one of them?

Answer 1

They know dead flesh, and they know alive flesh. ..

Answer 2

Go watch Shaun of the Dead and you'll understand.

Answer 3

Buy the smell.  

Answer 4

they can smell living flesh.

Answer 5

they wont know and they arent real  

Answer 6

Because if the writers had the Zombies attacking each other, the hero of the movie would not have to do anything.