Cross-Eyed Fears
All kids love to make funny faces but sometimes…well, most of the time they go overboard with it. Then, parents have to end up making a crazy lie to get them to stop because simply asking them to stop rarely works. Sometimes, even yelling at them doesn’t work.
So, this OP’s mother decided to avoid the drama and told him that if he went cross-eyed and someone slapped him in the head, he would be stuck like that forever. As you can imagine, this got him to stop doing it almost immediately. We can only imagine that after some time, it becomes really easy to make this stuff up.