I used to babysit for a little girl, Katherine, and when she was about 2 1/2 she would say that "this is mys" - it made perfect sense to her: if something that belonged to you is "yours" then if it belonged to her, it must be "mys".
A childhood belief about speaking

others believed things like this
- When I was little, I thought there were tiny cameras everywhere and they belonged to the government.— Brooke
- I used to think that the priest at church was JESUS.
He had a beard.... it made sense— Joe B - I used to believe that broccoli grew on trees. It looks like a tree, so it made sense, right?— Catherine...
- when i was 2 or 3 believed that i had lots of animals in my tummy it made sense my mum had baby in hers.— kjc
- I used to think that Batman was Blackman. He wore a black superhero outfit, so it made sense to me.Anon
- I thought Scott Joplin and Janis Joplin were related. Two musicians with the same last name - it made sense!— Rita