This is a skill I developed after weeks of practice. To do it, you put your hands together as if you were going to shake hands with yourself, with both thumbs facing you. It usually helps if your hands are a little damp; putting them in your pocket or sitting on them to make them sweat a bit will usually achieve the desired effect. The idea is to create a flow of air - try to suck air in from the bottom of your grasp, into a pocket in the center of your palms. Then, create a small exit point between the thumb of your right hand and the index finger of your left. Adjust the size of the opening until you get the fart noise. You'll never need a whoopee cushion again. This is great fun while standing in large groups of people, because you can make the noise near someone's hind quarters and watch as their friends gasp in horror! And since all you had to use was your hands, you'll never be suspect!

I've learned to do this well enough that I can now "scat", creating some pretty funky beats. I have three friends who can do this also, so when we all can get together, we create a fart-noise quartet. I know, pretty pathetic.