I don't think any of the other writeups actually explain the concept clearly.

The way I think about it is:

  • Any time energy is used to do something, some of it gets wasted due to the laws of thermodynamics. For instance, when you push your mouse across the mousepad, some of the energy is wasted on the friction of the mouse on the pad. This energy is bled off into the surroundings as heat.
  • Due to the pesky laws of thermodynamics, this waste heat cannot be used for useful work. In order to use heat to do work, you need to have heat move from something hot to something cooler. When the whole universe is the 'hot' source, where do you move it to?
  • And because the total amount of energy + mass in the universe is constant, and there's always some energy being bled off as useless heat energy,
  • Eventually, everything becomes useless heat energy

    Happily, all this will happen so far in the future, neither you nor your kid's kid's kid's kid's kids will have to worry about it.