Imagine that you love pistachio nuts and are given a room filled 5 feet deep with them. But you must eat them in the room and must leave the shells. When will you have eaten them all? Never. Because as it becomes increasingly difficult to find nuts amidst the shells, the cost of the nuts, in time and effort, will become too high. You will seek a substitute -- pistachios from a store, or another snack.
The point of this allegory is that we will never, ever, run out of any finite natural resource.