They can boil old nasty "gray water", or they can make a desalinator and remove the salt which works kinda like a moonshine still by boiling the water and then cooling the steam down and out the other end comes fresh water, or they can catch rain water.
The process of indentifying the benefits and costs of different alternatives by examining the incremental effect on total revenue and total cost causes by a very small (just one unit) change in the output or input of each alternative. Marginal analysis supports decision-making based on marginal or incremental changes to resources instead of one based on total or averages.
