Source: The New Games Book
Start with everyone standing in a circle holding hands. Now one of you gives a quick squeeze to the hand of the person on your right. This gets passed along to the next person and before you know it, you've got your original squeeze back again, in your left hand.
Keep passing it on until the squeeze is travelling smoothly around the circle. Now speed up the action a bit and add sound. Squeeze and say, "Ooh" and watch it go around. Next add "Ahh," but send it in the opposite direction.
Now try reversing the flow. When someone gives you an "Ooh," instead of passing it on, pass it back to them.
After that, you're just about ready for the ultimate: Freeform Ooh-Ahh. Start a sound, gesture, or combination of them going around the circle. Once it's rolling, transform it: add another action or change a sound.