With luck, you'll never need to know how to catch a fish without tools, but if the time comes where you're hungry and confronted with a stream full of fish without means to catch them, this trick might help. It's not impossible to catch a fish with your bare hands — it's just a little tricky, and Wired has a great guide to doing it.
Wired's guide covers how to best approach the fish you want to catch (downstream, from behind, since they spend most of their time facing and swimming upstream) and getting your hand in the water without startling it — slowly and from behind. If you do it right, you'll have one shot at scooping it out of the water, closing your thumb and index finger quickly to nab it by the tail if you don't get a good grip around its body. A friend of mine suggested skipping the approach entirely and dangling your arm in the water near a tree — like a part of its root system — and letting the fish come to you before scooping it up.
Obviously we're being more than a little tongue in cheek here — after all, there are few situations where you'll really need to tickle a fish out of the water bare-handed. Still, it's a survival skill that may come in handy for emergencies, or showing off to friends on your next camping trip. Just be careful and don't get bitten, okay?