I don't know

I was brushing my teeth. Fiona was explaining to me about the little mirror we have and why she had left it on the floor of the bathroom. Then she asked me what the mirror was for. I don't like to talk around the sonic toothbrush - too messy - so I shrugged my shoulders.

"You don't know?" Fiona asked. I shook my head.

"Why don't you know?" Fiona asked. I felt that I could just stick with the line I'd started, so I shrugged again.

"You don't know why you don't know?" I started to see where this was going, but I shook my head again.

"Why don't you know why you don't know?"

At this point I was laughing so hard I had to turn off the toothbrush.