system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Magnetism allows objects to affect the velocity of other objects (pulling or repelling them). Where does the energy that creates that force exactly come from, since energy can't be created nor destroyed?

I've been wondering this since I was five.
gpt
In order to move the magnet and the thing it's pulling on to a place where they'll interact, you have to do work *against the magnetic field*.  
If you put a bowling ball on top of a step ladder, you're doing work against the gravitational field.  
When the bowling ball rolls off and squashes a pumpkin or something, the energy comes from the work you expended putting the ball on top of the ladder.  

It's the same thing with magnets.  A magnet in a room has some potential energy with regards to all the magnetic things and fields in the universe. Physically moving the magnet to that place is like putting the ball on the ladder.  

Hope this helps.