You've probably seen someone do this before, perhaps while hailing a
cab: put two fingers in his mouth and let fly with an
ear-splitting shriek, handily catching the attention of the taxi driver, not to mention everyone within a block or so. You've probably wished that you could do the same (assuming you don't already know how). It's a reasonably simple thing to do, requiring only a small amount of practice; I myself learned it in under 10 minutes.
Step 1. Place the tips of your index fingers together. The proper angle differs from person to person, but it should be somewhere in the neighborhood of 45 degrees. Experiment a little to find the right position for your hands and mouth.
Step 2. Place your fingertips on the tip of your tongue (your hands are clean, right?). Push your tongue slightly back and upwards until your lips close on the top knuckle. Practice this while looking in a mirror if you're having trouble.
Step 3. Tighten your lips over your fingers and teeth until the only opening is in the V between the ends of your fingers. This is probably the hardest part to perfect, since you can't see what you're doing.
Step 4. Exhale steadily. You'll probably need to fine-tune the position of your fingers, tongue, and lips to produce that marvelous shrill shriek.
This can be done with any pair of fingers that can form the proper angle. Try it with the middle, ring, and pinky fingers, and, if you really want to show off, with the index and pinky fingers of one hand.