Why do tears come to my eyes when I yawn?

When people yawn, they tend to tightly shut their eyes, and this does two things. First it squeezes the lacrimal duct, causing more tears to flow into the eye. Secondly it squashes closed the tear ducts that drain the tear film from the surface of the eye. As a result there are more tears in the eye and nowhere for them to go. So when your yawn ends and you open your eyes again they are watery.