A free #5-weight floating fly line goes to the best answer to this riddle:

Nate and Sid went fishing with their 5-weights on a freestone mountain river. It was a clear, windy day, but they soon noticed a modest hatch of blue-winged olives concentrated in an eddy next to a big riffle with a deep tailout. Nate yielded Sid the “sweet spot” on the run, and so Sid went to the head of the riffle and tied on a size #18 BWO dry fly. He caught one fish. Then he looked over his shoulder to see Nate downstream, snickering as he was reefing in one brown trout after another. What was Nate using, and why was he catching more fish?