When approaching a trout stream, even those clear, easily spooked chalk streams, the fly you choose is based on sound, observational logic. You select the food item most likely to take the trout. If the water is covered in mayflies, you tie on something that approximates their appearance. Even if the water is blank, a good nymph or a juicy terrestrial presented in a lifelike manner will take fish. The trout angler, often armed with a knowledge of entomology sufficient to teach it at the college level, can almost always choose a fly that is logical. The trout eats stonefly nymphs, so I will choose a fly that imitates a stonefly nymph. I will work the fly in the water to the best of my ability to further imitate that stonefly nymph. If all goes well my fly will look and move like the real thing, thus inducing a strike.
Steelhead are far less cut and dried. Why steelhead take flies is one of the great mysteries of the natural world. Convincing an animal to put something in his mouth when he no longer has any interest in feeding seems like a fools errand, however it is precisely this unlikely venture that makes steelheading the great sport that it is. This begs the questions, what fly do you choose to make a fish eat that has no interest in eating? What is appetizing to the anorexic? That is obviously a facetious and silly question, but its a question the steelheader must ask himself on each and every trip to the river.