Reading off a page is like looking down at a landscape from a balloon – your eye "sees" the story as well as reads it, its layout, its paragraphs and structure, and "remembers" what it just read because it's still there, on the page, simultaneously. If you want to, you can reread any line instantly; or linger; or speed up; or optically "flinch." Reading a series of tweets is more like looking through a narrow window from a train speeding through a landscape full of tunnels and bands of light and dark. Each tweet erases its predecessor.
