When Andrew was about two years old he tried buttermilk at my Mom's house. Of course he didn't like it. For some reason he had it in his mind that buttermilk was called "just plain milk". I honestly have no idea why. But the morning after that visit when I was fixing his breakfast I asked him if he'd like juice or milk, and he asked what kind of milk.
"What kind of milk? It's just plain milk," I answered. He refused to have any. After a lot of questioning I figured out that he thought it was the kind he'd tried at my Mom's, so I had to pour that milk out, wash the glass, and say, "Ok, well, I'll get you just milk. It's the kind you drink every day. See? It's JUST milk."
He drank it.
A few weeks later he went to spend a few days with my parents. The morning after we left him there my phone rang. It was my Mom.
"What kind of milk does Drew drink?" she asked.
And I burst out laughing. I knew! "Oh no! Did you tell him it was 'just plain milk'?"
She chuckled. "Yes, I did, and he refused to drink it. I poured it back. What should I have called it?" (She's a wise old bird, my Mom.)
I explained, and for the rest of his visit he had JUST milk.