A runny nose, also known as rhinorrhea, occurs when excess mucus is produced in the nasal passages. This can happen due to various reasons, including allergies, colds, or sinus infections. The mucus may be clear, yellow, or green, depending on the underlying cause. When the body detects irritants or infections, it responds by increasing mucus production to help trap and eliminate these substances. A runny nose is often accompanied by other symptoms, such as sneezing or congestion, and is a common sign of conditions like allergies or the common cold.