Have the group lay flat on their backs.
When you say ÛÏfire on the mountainÛ the group is to stand up as fast as possible. The last one up has to then sit out until the end, or do ten jumping jacks, push-ups, sit-ups etcÛ_
When the group is on their back they are to lie perfectly still. If you say something other then mountain (Mickey mouse, Montana, Mazda etcÛ_) and they flinch or begin get up then the sit out or they do the jumping jacks as well.