How do people beome cross-eyed? As far as I can tell it is not possible to move your eyes independently of one another…
You can move eyes independently of one another. A friend of mine and a poster here Micilin can move one eye in then the other and the moves one out and then the other. It looks quite wierd .
As to the OP.
[WAG]A problem with the muscles that control the eyes. [/WAG]
The opti-grab that Steve Martin invented in The Jerk did the trick aswell 
And welcome to the Straight Dope .
Thank you. I tried to do it but couldn’t.
IIRC, it’s not a problem with the eye muscles being to weak to move each eye independantly, it’s that they’re too strong. People who can move their eyes one at a time (myself included, I have quite the show) classically have very strong astigmatism, or are very nearsighted. Their eye muscles are weaker than normal, which means they can ‘override their programming’ that holds the eye in place.
–Tim
Three of your twelve cranial nerves are responsible for moving the eye. One of them, the abducens nerve or CN VI, innervates the lateral rectus muscle, whose job it is to abduct the eye (that is, to turn the eye away from the nose).
When something damages the abducens nerve, you lose the abduction, so the eye is adducted (the opposite of abduction; that is, turned in toward the nose). If this happens bilaterally, you get crossed eyes. (This is often referred to as “sixth nerve palsy”.)
On a side note, a bundle of nerves known as the medial longitudinal fasciculus is responsible for coordinating eye movements; that is, if my brain says “look to the left”, the MLF makes my left eye abduct while my right eye adducts. If you damage the MLF, you lose this coordination.
(Eye movements are extremely high-yield material in the first two years of med school, since they lend themselves to good multiple-choice questions.)
Dr. J
But is it possible to consciously over-ride this without having nerve damage?
A family friend had a weak lateral rectus muscle. She had to concentrate to keep looking forward. But if you were talking to her casually and she relaxed, her right eye would suddenly point to her nose. Quite freaky.
(Or am I weird?)
Instructions:
Cross your eyes by bringing them together to look at the tip of your nose.
Now you can move one eye out while the other one remains. Bring it back in.
Repeat with the other one.
Do a few times and you will see it is quite easy.
I can cross my eyes (really cross, not looking at the tip of my nose), look to the left, re-cross, look to the right, and so on. This looks sort of like moving them independently, though I can’t pause in the middle. It also gives me a headache after a while. If you want to try it, put your finger about a foot in front of your face, then slowly move it in to the bridge of your nose, focusing on it the entire time. Only a few people can do it.
Mom always hated when I did that, so she asked the doctor, who said it meant my eye muscles were strong. Of course, that meant she never asked about my double jointed shoulder. 
::sigh::
How do people become cross-eyed? I was born cross-eyed. I had two surgeries before age 3 to correct it as much as it could be corrected. (Then, you throw in my astigmatism, and you know why I’ve had glasses since I was four or five.)
After that, it was a matter of getting the eye muscles stronger, and reducing the dominace of my left eye. When I was about seven or eight, my opthamologist had to wear those sticky bandages as eyepatches, to force me to use my right eye. I totally hated it. It must have worked a little, though; my left eye is still the dominant eye, but not as much as it used to be.
What’s it like to be cross-eyed? It sucks. I can’t completely focus on something with both eyes. If I try to, I wind up seeing more periphery than what I’m trying to focus on. I also used to get a lot of nasty eye/brain fatigue headaches (at one time, as often as once a week) from the constant focusing and refocusing.
What sucks the most is that often, the non-dominant eye floats. People look at you funny, because they don’t know which eye to look at. My eye doesn’t float as much as it used to because I have better control of it now. Maybe having contacts has helped, too. When I’m tired, though, it tends to float back up.
Taking decent pictures can be hard because of the floating problem, but again, with age, I’ve gotten better at controlling it.
Of course, the difference between making crossed eyes and having crossed eyes is that as soon as you’re done taunting someone, your vision goes back to normal. Mine doesn’t.
DoctorJ, thanks for your post. That’s more than I got out of years of going to eye doctors.
I hope you didn’t think I was making fun of you. It’s just a odd body capability, like touching the tip of your nose with your tongue or wiggling your ears.
I took no offense at all, Pigs in Space.
(BTW, I found it impossible to type that without laughing. It’s quite a non sequitur.)
The strange thing is if I try to cross my eyes to tease someone, I can’t. I don’t know why. Psychological resistance to doing something I ordinarily don’t want to do? ::shrug:: You got me.
In case anyone’s wondering, my crossed eyes aren’t noticeable. I either have to point it out to people (yeah, like I’m going to do that!), or, if looking intently at someone, fail to consciously maintain even eye contact. Otherwise, no one can tell. Thank God.
I’m glad. Better to ask…
Yeah. Well, if you have any other questions… please do ask.