If you cross your eyes and someone hits you in the back, your eyes will be frozen like that.
Chris first remembered hearing this from his mother in Spanish. He was unsure of his exact age but knew it was as a little kid. Older people mostly tell this item to younger children, as a lesson. Adults would almost threaten the children, saying their face would stay that way permanently, thus encouraging them to not make silly faces. Because of his age, Chris obeyed his mother and did not make any more faces. Chris also is not sure whether of not this superstition is Mexican, the descent of his mother, or just a general one.
I also remember hearing this superstition, making it most likely not specific to Mexico. I heard it growing up in Hong Kong, showing that the item has traveled over the years. The version I heard was a little different though. The one I am accustomed to hearing is that if you make a funny face and the wind blows, your face will stay like that permanently. At first, this really scared me but it definitely didnt stop me from making the faces and seeing if it really would freeze.
Once becoming a teenager, most people realize there cant be any truth to this superstition. This is why it is more popular with little kids. They are always running around, getting in trouble, and doing silly things. It is a way for parents to tell their children not to do something without getting a multitude of questions asking why. By simply saying this will happen if you do this, kids will be scared and comply. If my child were making a funny face at the dinner table or another inappropriate place, I would probably tell them the same thing. Children are very gullible and believe anything they hear, especially if it means their friends will make fun of them. At the end of the day this superstition cant be taken too seriously, but it is still amusing to tell.