First, living bugs move-they commonly hop, dart, flutter, or twitch on the surface. Second, trout are attracted to such movement. A trout's survival depends on its ability to differentiate between drifting food and the countless bits of inedible debris also carried in the current. It does so, in large part, by detecting movement. You can use that to your advantage by skating, skittering, and twitching your dry flies. The key is to know when to do each and how to do them in a way that is most apt to draw a strike from a hungry trout.