When you yawn, why do your eyes water?

Yes, it is. Part of the reason is that when you yawn, you squeeze your eyes tightly shut. The way that tears flow is that they come out of your lacrimal gland, which is on the upper outer side of the top of your eye; they then flow - in a film - across your eye obliquely downwards and inwards. The tiny black dot on your lower eyelid is called a punctum, and that's where your tear duct starts and where the tears drain away. But if you squeeze your eyelids tightly shut, you close off the punctum, stopping the tears flowing across your eye and into the tear duct, so they build up in the eye. This makes you cry a little bit, which is why tears appear when you yawn.