(Intro)

So you're on a first date, skating at your local outdoor ice rink on a frosty winter evening. Your partner grabs you by the hand, looks you lovingly in the eyes and sees one wet runny booger coming out of your nose. You sniffle and snuffle but your nose just won't stop running.

When it comes to your runny nose embarrassments you can blame both biology, which makes your body produce more mucus, and physics, which turns some of the water vapor you inhale into liquid.

On the biology side your nose starts producing more mucus to warm and moisturize all that cold, dry air. Some of the mucus drains out your nose for the whole world to see in what doctors call rhinorrhea. The mucus dripping from your nose comes from your mucus membrane, the thing protective blanket of cells that covers the inside of your nasal cavity. The mucus is made mostly of water, salts, white blood cells, and glycoproteins, or proteins that are attached to carbohydrates. When the air gets colder and drier, like in the winter, your mucus membrane is triggered to secrete more mucus which moisturizes the air so your nasal cavity doesn't dry up, crack, and bleed. As a bonus the mucus traps particles in the air, stopping a lot of the harmful stuff from getting into your lungs.

Now after you inhale you end up with two very nice, warm lungfuls of air ready to be exhaled which is where the physics part comes in. Some of what you exhale is water vapor which has been suddenly pushed from a nice, warm environment to a harsh cold environment. That sharp change in temperature causes the water vapor to condensate into little water droplets on the tip of your nose, the same way that water condensates on the cold bathroom mirror while you're taking a hot shower.

All of a sudden you've got water and mucus dripping from your nose and this date is just not going very well. Next time just go bowling.

Thanks to Patreon patron Heather Thornburn for asking this question and thanks to all of our patrons who keep these answers coming. If you'd like to submit a question to be answered just go to patreon.com/scishow. And don't forget to go to youtube.com/scishow and subscribe.