A few weeks ago I was having a discussion with a colleague of mine. We were chatting over different writing styles we see in student work, as we both teach the same students. I teach them in a writing class, and she teaches then in an English class. She mentioned how she finds it very difficult to teach "voice" to students. An instructor can go over many conventions and strategies as well as provide different writing activities to help students develop voice in their writing. But she said some students just seem "to have it" while other students really struggle. It is almost like it naturally and almost effortlessly flows from some students into their writing.
