A nostril is one of the two openings at the bottom of your nose. When you breathe, air flows in through your nostrils, travels up into your nasal passages, and continues down to your lungs. Your nostrils are the entrance to your respiratory system.

Most people don't think much about their nostrils until they get a stuffy nose from a cold. When one nostril gets blocked, you suddenly notice how much you rely on both of them for comfortable breathing. Your nostrils also play a key role in your sense of smell: when you sniff something, you're drawing air quickly through your nostrils so scent particles can reach the smell receptors inside your nose.

Animals have nostrils too, though they might have them in different places. A horse's nostrils flare wide when it runs hard and needs extra air. A dolphin has a single nostril called a blowhole on top of its head. Dogs have incredibly sensitive nostrils that can detect scents humans would never notice.

People sometimes wrinkle their nostrils when they smell something unpleasant or flare them when breathing heavily after exercise.