Answer:
Can and May
Children in American schools learn to use the modal may when asking for permission. A student might ask the teacher, "May I be excused?" before leaving the room. When students asked, "Can I leave the room?" their teachers often made a joke, "You can, but you may not."
The teacher was saying the student is able to leave the room, but does not have permission to do so. May is the most formal way to ask for permission. The distinction between can and may is slowly disappearing in English.
