HOW COME?: Our noses run in winter

How come in cold weather your nose runs and your eyes tear? Also, how come when you put milk on puffed rice cereal, it makes a popping sound? asks Emily Abrams, a student at Long Island Lutheran Middle School.

Winter is notorious for its dry air. And yet, when it comes to noses and eyes, cold weather seems to bring out the moisture.

If you've ever taken a walk on a frigid day in the biting wind, nose dripping and eyes watering, you've experienced the (annoying) reality of "Jack Frost nipping at your nose."

According to doctors, our eyes water because frigid air and wind irritate the covering of the eye, stimulating the tear glands to step up production. So being out in the wind on a very cold day is a bit like having sandy grit blow into your eyes.

Runny, cold noses, however, are a bit more complicated. It's the job of the nose to warm the air we breathe before it's inhaled into the lungs, and to make sure it's moist, too. Air enters the nose and picks up moisture from mucus. The incoming cold, dry air prompts the nose to increase its production of fluid, and extra mucus flows.

Meanwhile, the air you breathe back out on a cold day is much warmer and moister than the air around you. Warm air can hold a lot of water vapor. But as air cools and gas molecules pack more closely together, air reaches its saturation point sooner. When the exhaled water molecules in your breath hit the cold air, the parcel of air in front of your face suddenly becomes "supersaturated" with water. The result: The water vapor immediately condenses into droplets, forming a tiny cloud. And you can actually see your breath.

But some of the moist air you're breathing out condenses into droplets at the tip of your nose. And these droplets of water mix in with the overflowing mucus. So after a few minutes in a cold wind, your nose is doubly runny.

Of course, you could just stay indoors - and listen to your cereal. Puffed rice may not be the warmest cereal on a winter morning, but it could be the loudest.

Rice Krispies and other puffed cereals complain noisily when doused with ice-cold milk. How does it work? Think of popcorn. Just as hard corn kernels are popped into popcorn, so rice, wheat, and other grains can be expanded into fluffier versions of themselves. In the case of puffed rice, cereal makers condition the rice with water, then toast it in a hot oven. As the trapped water turns to steam, rice kernels puff out like microwaved popcorn.

Unlike the hard, compacted walls of an uncooked rice kernel, the walls of puffed rice are stretched thin, making each kernel very fragile. Pour on the cold milk, and the shock causes the walls to crack like a thin-walled crystal glass. As the milk is absorbed - unevenly - by the puffed rice, we hear the sounds of fracturing walls and escaping air bubbles, freed from inside the kernels. Snap. Crackle. Pop.