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Have you ever noticed that when you use eye drops or put on mascara, your mouth hangs open? Although there aren’t any hard numbers on how many people experience this “mascara mouth” phenomenon, it’s definitely pretty common.

Just watch next time you’re around someone putting in their contacts. So why is it that messing with our eyes often leaves our mouth agape? No one knows for sure, but there are some best guesses as to what might be causing it.

We can start by looking at the nerves that control facial movements — of which there are quite a few. The trigeminal nerve is a large, complex nerve that controls the lateral pterygoid muscle, which along with other muscles, opens and closes your mouth. And the oculomotor nerve is responsible for controlling a wide range of muscles in and around the eye -- including the muscle that raises and lowers your upper eyelid.

Although these nerves do pretty different things, they originate close to one another in the brain stem. So one hypothesis to explain mascara mouth is that the two nerves are so close together, they get their signals crossed. When you activate your oculomotor nerve and open your eyes wide, this might also accidentally trigger your trigeminal nerve, causing your mouth to open.

Some researchers have made a connection to this response with a very rare genetic disorder called Marcus Gunn syndrome. This syndrome causes one eyelid to move rapidly every time the jaw moves. It’s thought to be caused by an abnormal connection between the trigeminal and oculomotor nerves.

And researchers aren’t totally sure why yet. But some think that it’s a more extreme version of this “mascara mouth” response that most of us seem to have. Which could help explain why individuals who don’t have this syndrome will still open their mouths while poking around their eyes.

Another interesting connection between our eyes and our mouths shows itself in a reaction called the corneomandibular reflex. This is an automatic, completely involuntary movement of the jaw which happens when touching the cornea, the transparent front layer of the eye. Touching the cornea makes you blink, and the jaw movement seems to happen in response to that.

It almost always shows up in patients with brain related damage or diseases, like those with Parkinson’s or ALS. It’s well known in comatose patients, suggesting it can happen when you’re totally unconscious. It’s thought that this reflex is present when there is either a brain stem dysfunction, or structural damage to nerve pathways — possibly another mixed message between the oculomotor and trigeminal nerves.

So while there are definitely a number of connections between our eye and mouth movements, we’re still really not sure what exactly is happening to cause your mouth to hang open when you’re trying to convince your stubborn contact lens to stick to your eyeball. What we do know is that it’s hard to avoid looking silly. Thanks to our patron Katy for sending in this question, and thanks to everyone who voted in our Patreon question poll.

If you want a shot at getting your questions answered or just want to vote in the polls, head on over to patreon.com/scishow. [ ♪OUTRO ].