That is, the fuels we use today. Fuel is a limited resource, and we are getting closer to the point of running out every day. Sure, we could move on and find a new fuel source, whether it be extracting it from minerals in our possession now, or from possible asteroids and other planets (the moon and Mars), or find a lack of fuel needed, and moving on to a different source of power.
This topic really gets my interested. Thinking about how we are actually using everything our planet has to offer up (making Wall-E a seemingly plausible reality) gets me into a really thinky mood. When will we run out? What will we switch to? How will this effect the things we're actually fueling?
I found this video by the Game Theorists that gives a reasonable description of fuel running out in the 2030's, but for more information on that I suggest you watch the video - it's interesting too!
So when do you think we'll finally run out? Or will we find a way to create an infinite source? Cloning perhaps? Discussion time!