One student guessed that a preposition was the subject of an example sentence, and my daughter gave a rather lengthy explanation about why that was not, in fact, correct.
I believe the sentence in question was something like:
The students went to the school.
"To the school is a location. It's where they're going to," she said. "So that can't be the subject. The action with the verb is where you find the predicate, and to the school is at the end of that, so it's part of the predicate. But the students are the ones doing the action. So the students is the subject."
The class stared at her in stunned silence.
My daughter closed her mouth, swallowed, and, looking around at the class of gaping fifth graders realized:
Oh my gosh . . . I'm turning into Mom.
Sniff. I'm so proud. What a great Christmas gift.