Setting the Scene:
1. One or more Scouts go some distance away from the campfire circle. When the skit has started they will advance in stages making noises like old Wild-West Indians ( I will use woo-woo-woo for this description
2. Two Scouts remain in the campfire circle, one of these is the guard and the other is the Captain of the Fort (the campfire circle)

Indians: woo-woo-woo
Guard runs towards the Captain shouting: "The Indians are coming, The Indians are coming"
Captain, enjoying a drink / nap, lazily asks: "How far away are they?"
Guard says: I don't know
Captain says: "Well, how big are they?"
Guard (holding hands vertically quite close together) says: "They're this big"
Captain says: "Oh don't worry man, that means they're miles away yet. Come back when they get closer."
Guard returns to his post and waits a second or two.
Indians: woo-woo-woo
Guard runs towards the Captain shouting: "The Indians are coming, The Indians are coming"
Captain, enjoying a drink / nap, lazily asks: "How far away are they?"
Guard (holding hands vertically quite close together) says: "They're this big"
Captain says: "Oh don't worry man, that means they're miles away yet. Come back when they get closer."
Guard returns to his post and waits a second or two.
Indians: woo-woo-woo
Guard runs towards the Captain shouting: "The Indians are coming, The Indians are coming"
Captain, enjoying a drink / nap, lazily asks: "How far away are they?"
Guard (holding hands a little further apart) says: "They're this big"
Captain says: "Oh don't worry man, they're still miles away yet. Come back when they get closer."
Guard returns to his post and waits a second or two.

repeat as many / few times as you think you can get away with until you get to the following:

Indians: woo-woo-woo
Guard runs towards the Captain shouting: "The Indians are coming, The Indians are coming"
Captain, enjoying a drink / nap, lazily asks: "How far away are they?"
Guard (holding hand up at head height) says: "They're this big - I can see their eyes now"
Captain jumps up and shouts: "What are you waiting for - shoot!"
Guard stands there looking sheepish and say: "I can't do that"
Captain says: "Why ever not?"
Guard (holding hands close together) delivers the punchline: "I've known them since they were this big!"


Think about it.....