In the daycare center where I work, the classrooms for the under-2 kids have chest-high doors opening out into a large common playroom. You can always see what's going on in the playroom and in the classrooms.
The other day, one of the front office staff members came and looked over my door. 3 little boys, all 20-22 months old, were seated at the table drinking milk. "That's Miss Callie," I told them. "Can you say Callie?"
One of the boys made a stab at it: "Cawwie," he said.
One of the others immediately said, "Moo."