I teach 10th grade chemistry (most students age 15-17).
1) We did a lab with pennies and one student asked me how to tell which side of the penny was "heads up."
2) We did a lab with popcorn and one student was shocked to learn that popcorn does in fact come from corn.
I teach 6th grade and I talked about the Catholic Church's sale of indulgences to absolve people of sins. At the end of the lesson, this student asked me "What is an an indulgence?"
Watching the Transformers movie for a little downtime, a student asks "... so.. This actually happened?"