How do grades work again?

Shelli flounces up to my desk one afternoon, obviously upset.

"I have to talk to you about my grade," she huffs at me.

"Ok, what's up?" I reply.

"It says I have a zero for one of my tests."

"Yes." I say, waiting patiently.

At this, Shelli stamps her foot (stamps her foot!) and heaves an explosive sigh. "So I don't understand how I can have a zero on a test I didn't even take!"

I look at Shelli for a second.

Or two.

"You want to rethink that statement for a little while and get back to me?"