There's a fish under the surface, trailing him. The fish wants to eat the fly, but the fly's too high up. The fish knows that if the fly lowers its altitude--probably half a foot or so--he'll be able to jump out and snap him up.
There's a bear on the shore, watching the fish. The bear wants fish for dinner, but the fish keeps darting around, following the fly. The bear's waiting for the fish to jump out of the water so he can grab him.
Across the stream, about a hundred yards down, a hunter has the bear in his scope. The bear's standing behind a thicket of dense shrubs, and the hunter can't get a clear shot. What would really help would be if the bear would lunge out from behind the shrubs to grab the fish.
The hunter brought his cat with him. The cat's been eyeing the hunter's sandwich all day. It's been a slow day hunting, though, and his owner has kept his sandwich in close reach. The cat's been waiting for the hunter to shift his attention so he can nab the sandwich.
Sure enough, the fly finally drops down that half foot.
The fish jumps out of the water, eats the fly. The bear plunges out from the shrubs and grabs the fish mid-air. The hunter aims at the bear's torso and fires three rapid shots. The bear drops. But instead of grabbing the hunter's sandwich, the cat gets startled by the gunshots and falls into the river.
The moral of the story is: When the fly goes down six inches, the pussy gets wet.