system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do animals have two nostrils?

I understand we have two eyes for depth perception and two ears for telling the direction of a sound, but what is the point of having two nostrils?
gpt
Nostrils were originally used to sense smells of small molecules dissolved in water. They didn't even connect with the inside of the mouth, but instead each one was a little u-shaped loop that water could flow through (many fish still have nostrils like this).  A nostril on each side of the head lets these animals tell what direction a scent is coming from, which is pretty useful if you want to follow it or avoid it.