The answer is entropy. Entropy is defined as the chaos of the universe.
But let me put it in simpler terms....
We have a nice little ice cube sitting in in a beaker.
It's nice and oddly shaped, but is still a solid.
Well over time this ice cube melts. You might say this is only because the melting point of an ice cube is 0C and that the ice cube is more than likely placed in a room at room temperature (25C). Nevertheless the ice cube has melted. It's molecules are dispersed around in the beaker moving freely in said volume.
Well how can I connect this to the world?
Well the world is like that ice cube, eventually the world will turn into a giant liquid and eventually a gas due to entropy. This will happen way way beyond our generation don't worry. So don't worry bout our sun dying out because the earth will die out first. And this is also how cars rust, things have experation dates, and why you will eventually die :D
Aren't you so glad that global warming is something we shouldn't give a damn about because our world will eventually disappear?
This is Entropy :D
Shows what the college board puts on the AP Chem exam also -_-
Lol yeah... Hopefully not in our generation. :P