Q. Why do yawns sometimes cause one’s eyes to tear?

A. The eyes are usually coated at all times by a film of lubricating tears, flowing from the lacrimal glands at the upper margin of the eye socket. The gentle flow is spread evenly across the eye by blinking and is eventually drained by two small openings at the inner corner of the eye — the superior lacrimal punctum and the inferior lacrimal punctum.

Yawning often increases the secretion of tears because of muscular tension on the glands, which may simultaneously dam up the drainage system. The result can be that the film builds up into a small waterfall of tears.

The involuntary act of yawning usually includes opening the mouth very wide while slowly taking in a deep breath. The eyes often close as well. The result is a temporary rearrangement of the facial musculature. Repeated yawning can multiply this tension.

The tear system eventually leads to the nasolacrimal canal, linking the eyes to the nose. Thus, a bout of yawning, like an emotional episode, can produce a runny nose as well. questions@nytimes.com