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.