system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If perpetual motion machines don't exist, what is powering the earth's natural systems? (Plate tectonics, water cycle, orbit)

And, as a bonus, will they run out of energy? what will happen?
gpt
Perpetual motion machines do not exist because energy has to come from somewhere.  However earth's natural systems are not considered perpetual motion because the energy always comes from somewhere.  Plate Tectonics are powered by the energy of Earth's molten core which moves to Earth's surface by way of cracks between the tectonic plates.  You see evidence of this in the volcanoes in islands in the oceans.  Hawaii and other islands were created by such lava flows that caused the island to grow bigger and bigger.  

The water cycle is powered by the radiant energy from the Sun that causes water to evaporate and rise into the air as steam.  When it goes high enough, it cools and become clouds and then condenses around particulates in the air and fall to the ground as rain.  the rain either re-evaporates into the air, or it sinks into the ground until it meets underground rivers and makes its way to the oceans and lakes where it evaporates there.  A wholly remarkable system, that.

edited to add:  The Water Cycle will not stop until it stops receiving radiant energy from our Sun.  This won't happen for billions of years when the Sun runs out of hydrogen fuel and becomes a Red Giant which will cause it to expand massively and engulf the Earth.  The Earth's plate tectonics won't stop as long as Earth's molten core remains liquid which won't happen unless Earth is broken free of the Sun's gravitational grip.  What this means is that when the Sun runs out of hydrogen fuel and becomes a Red Giant, Earth's molten core will *still* be liquid.  It will simply be vaporized when the Red Giant engulfs earth.