Thomas Jefferson and several modern writers used fire as an analogy for copying intellectual property: we can light many candles from one without diminishing the light or heat obtained from the first candle. Suppose a group of people go camping in the wilderness using primitive methods. One person gets a fire started. Others want to start their fire from hers. Give ethical or practical reasons why they might be expected to trade something, perhaps some wild fruit they found, for the use of the fire?