Clearly, a third element of writing is the topic or subject. Again, this element seems straightforward. We must have something to write about. In practice, though, what we say or write depends not only on what we know about the topic, but also what we assume the reader knows about the topic. Let me give you a brief example. If I write a brief e mail to an old friend about a hike we've planned, I can be cryptic because we've hiked together before. My message might be as short as "meet at trailhead at 7:30; my sandwiches, your drinks." But if I'm inviting someone new to the area to go hiking with me for the first time, I'll have to be much more specific: "I'll pick you up at 7:00. This trail is often muddy near the lake, but the fishing should be good so I'll bring an extra pole for you. Bring a lunch and plenty of water; I'll bring two rain ponchos." Because I can't assume that this newcomer has acquired fishing and hiking equipment, and may not even know about not drinking from streams and lakes, I have to be more explicit about the supplies we need to carry with us.