"What's a bottle cap?"

Recently, Micah and Josiah have started asking "What's a ..." whatever happens to be mentioned, or whatever they happen to see. They'll ask, "Mama, what's Noah's name?" What kind of a question is that? Annoying, that's what it is!

Well, I was in the kitchen cleaning up, when I knocked over a plastic container full of bottle tops into the kitchen sink. "Oh, shoot!" I said.
"What happened?" Micah wanted to know.
"I spilled the bottle caps in the sink."
"What's a bottle cap?" Micah asked.
I showed him the container with some of the caps inside and said, "This is a bottle cap."
"What's a bottle cap?" Micah repeated.
"This is!" I replied. "It's a bottle top. It's to close the bottle. This is a bottle cap!"
"What's a bottle cap?" Micah continued.
"Micah," I said, losing my patience, "I just told you what a bottle cap is. I showed you what a bottle cap is. This is a bottle cap. You know what a bottle cap is. You don't need to ask anymore what a bottle cap is, so don't say, 'what's a bottle cap', because you know what it is, okay?"
Micah paused to take in everything I said, and then, "What's a bottle cap?"
I couldn't help it. I just started laughing. Then Micah asked, "What's so funny?"