It’s the first sense you use when you’re born. 95% of your nasal cavity is used to filter the air you breathe in before it hits your lungs.
Your brain has 40 million different olfactory receptor neurons. All of these different combinations of neurons let you detect an array of different smells.
Not everyone smells the same things. Take asparagus pee for example, only 25% of people can smell the distinct odor of asparagus pee. The other 75% of the population don’t notice this pungent smell.
Your olfactory neurons are the only neurons in the body that gets replaced regularly every 4-8 weeks.