They stand in a circle.
Someone starts and using their thumb (like hitching a ride) passes to the right saying Zip
It passes round the circle until someone says boing (which is like refusing it)then it travels the opposite way with them saying Zap
If they get really good she can introduce Kapow/ker bluey/??? and then it travels accross the circle to the opposite side (and that person decides whether it is Zip or Zap (or Boing passing it back to the person who sent it)