"No," I said.
"It's when you sing but you don't sing," she explained.
"Oh, you mean like this?" And I sang "Gaudeamus" with la, la, la.
"No," she objected.
"Then, do you mean humming?" and I hummed a few bars.
"No!" she said. I was bewildered.
Then I said, "You mean, when you hear the song just inside your head?"
"Yes!" she answered. "Now, you quiet-sing 'Gaudeamus' and I will too." We sat in silence for a few moments. Then she said, "Now, 'Lullay'" and we sat some more. I was dutifully singing The Coventry Carol in my head, when she called out, "'Don't Fence Me In!'". I complied. In a moment she said, "Are you singing 'Don't Fence Me In'?" I said, "Yes, you told me to." She nodded. "Now, sing 'Gaudeamus'.
I said, "You mean, out loud?"
She said, "Yes." I sang 'Gaudeamus Hodie' through a few times and she was asleep.