An actual conversation with a student during one of my classes last week:

My student said something in French and I answered him in English.  Then this happened:

Student (in French): Wait. She understands French?
Me (in English): I live in France.  Of course I understand French.
Student (in French): But if she speaks French why is she making us speak in English?
Me (in English): ...Because this is English class.
Student (in French): Oh. Right.

Sometimes I wonder about these kids.