question

A runny nose is normally caused by streaming eyes draining through the tear ducts into the nose. The watering eye’s response is mediated by the trigeminal nerve, which is the main facial nerve and has branches in the mouth, nose and eyes. The response probably evolved as a way of flushing the eyes and nose of irritants. With hot food, that irritant is the capsaicin oil; in cold weather, the drying effect of the wind is to blame.