Have you ever wondered why your nose runs when you are breathing in cold air? Well this is what happens inside your nose. You have to understand that on an average day a person produces almost under one litre of mucus. Eww! That is a lot of snot (another name for mucus). So most of this slides back into our throat. All this happens without us knowing most of the times.
Now, when you are out in the cold air, the blood vessels in your nose dilate and increase the blood flow. This happens in response to breathing cold air. It does two things, first it keeps your nose warm so that it can breathe easily and second it warms up the cold air before it reaches the lungs.
When this blood supply increases in the nose, it also reaches the glands that supply the mucus. Higher the blood supply, higher the mucus. So when the mucus is produced at a higher rate than normal, your nose starts to run. When you go back to normal temperature the blood vessels constrict and the blood supply decreases producing lesser mucus. Your nose stops running.
Now tell me why do you get a runny nose when you have cold or allergies? When you have a cold, your body produces more mucous to help keep out the germs. The nose running is just an excess of mucous.
Okay can you guess why your nose runs when you eat spicy food? It’s not very clear why certain foods cause our nose to run when we eat them. It seems that some spices specially red chillies trigger the brain and it sends out a message to the mucous membranes in the nose to produce more snot. Remember to keep a tissue handy next time you gorge on spicy hot food :)