As all fly fishermen know, most of the time trout feed below the surface. To catch them involves offering nymph imatations subsurface and while doing so most fly fishermen use an "indicator" on the surface.
Granted there are those purists that feel that is no more than a bobber, but the bottom line is that it is a great aid in catching trout on the fly. Indicators can take various forms, from a large dry fly, to a hank of yarn, to an actual bobber (well, a "Thingamabobber", which, since its introduction a few years ago has quickly become the indicator of choice for the majority of fly fishermen.)
If you should find yourself without any Thingamabobbers, but have some yarn, you can use the following two knots to tie the yarn to your line for indicators:
The Open Loop Knot or The Double Uni Method. Both work well, though I'd give the nod to the Double Uni Method for reasons explained on its page.