system
Given the following post, respond as if you're commenting on Reddit:
human
Why does my nose get watery when I eat something really spicy?
gpt
Your airways are constantly producing very large glcyoproteins (proteins covered in long chains of sugars) called mucins. These are very sticky, water-binding molecules that function to protect the airways. Mucins are generated by specialized cells in little "bubbles" in the cells. These bubbles are prepackaged and stored. When mucus is needed, the "bubbles" are transported to the cell surface where they fuse with the plasma membrane and dump their contents out into the airways. The quantity and type of mucins released depends on where you are in the airway, and the physiologic state (irritated, infected, inflamed, etc) of that region of airway. 

But these are very thick, viscous proteins. If they are released by themselves, you would get a very thick and viscous goo. So, packaged in these "bubbles" along with the mucins are other molecules that stimulate the release of water into the airways. The water functions to dilute the mucus. The quantity of water is also regulated.

So you have two parts to mucus production: The release of mucins and the release of water.

The function of mucus is to keep the airways clear of foreign material. So, as you might imagine, one of the things that will cause localized increased mucus production is the presence of a physical object (e.g., a speck of dust). This response is mediated (in part) by nociceptive (damage-sensing) sensory neurons. 

When these neurons fire, they stimulate the specialized mucus-producing cells to release those little "bubbles." This stimulation is regulated, and will only occur in the region where the object has been detected by the sensory neurons. This results in the release of mucins and the release of water into the airways. 

Capsaicin has the ability to "short-circuit" these neurons, making them fire. 

This is essentially a drug effect; you are taking a drug that causes your body to respond in a way that it normally wouldn't. 

There are two main differences in the capsaicin response and the "normal" response. First, capsaicin will not be a localized effect. When you eat a chili pepper, some of the capsaicin is aerosolized and gets spread over a relatively large area of the airway. Second, capsaicin is kind of oily...it is more soluble in fats than it is in water. So, when you try to wash it away with a water-based rinse (like mucus), it is not cleared very rapidly. Therefore, pushing water through the area where capsaicin is won't do much to clear the capsaicin. So the effect is prolonged.

So capsaicin gives you a longer-lived response put over a larger area.

Mucin is hard to make. It's a big protein, and making big proteins takes time. Pushing it out is quick. The molecules that stimulate water production are small and easy to make, and often stored in very large quantities. In fact, one of these molecules is ATP (yes, the same ATP), and the cells have huge quantities of that in reserve. You deplete the mucin relatively quickly. You don't really deplete the water secreting part.

The end result is that you get a sudden depletion of the mucin component of mucus, but the water secretion keeps going....and you get a thin, watery mucus.