Student: Can I borrow a pencil?
Teacher: I don't know. Can you?
Student: Yes. I might add that colloquial irregularities occur frequently in any language. Since you and the rest of our present company understood perfectly my intended meaning, being particular about the distinctions between “can” and “may” is purely pedantic and arguably pretenttious.
Teacher: True, colloquialism and the judicious interpretation of
context help us communicate with nuance, range, and efficiency. And
yet, as your teacher, my job is to teach you to think about
language with care and rigor. Understanding the shades of difference
between one word and another, and to think carefully about what you
want to say will give you greater power and versatility in your
speech and writing.
Student: Point taken. May I have a pencil?
Teacher: No, you may not. We do not have pencils since the state cut
funding for education again this year.
