Confusion between Force and Energy
Many, many questioners at the Wikipedia reference desk wonder why either magnets or gravity don't "run down" and since they don't, they'll go on to ask why you can't use the "energy" of gravity or magnetism to make some kind of free energy/perpetual motion machine.
The problem is that these folk are confusing "force" with "energy" - and you can be forgiven for making that mistake because (it seems) nearly everyone does!
A magnet exerts a force - it does not "produce energy". Your fridge magnet will stay stuck to your fridge until hell freezes over without "running down" because it doesn't need to expend energy to do it. Energy is produced or consumed when something that's exerting a force moves. So when you peel the magnet off of your fridge, it costs your muscles energy to do it. When you release the magnet from a half inch away from the fridge, it releases that energy by moving back onto the fridge - and the energy you added with your muscles is turned into heat and sound as it snaps back onto the metal. The magnet itself doesn't HAVE energy to give away - it's not like a battery or a wound-up clockwork toy.
The clockwork toy is a great analogy. If you wind up a clockwork toy car and place it on the table with your hand in front of it, you can feel it pushing against your hand. But until you move out of the way and let the car shoot across the table, it's spring won't ever run down. It'll stay there with the spring wound up, actively pushing against your hand, until you release it - even if that takes an hour, a week or a thousand years. Once the car starts moving, it gains "kinetic energy" (the energy of motion) and gradually loses the energy in the spring.
The situation is exactly the same as with another common force...gravity. If you place a book onto a table, the book and the table are exerting forces on each other - the book is being pulled downwards by gravity - and the table has interatomic forces that resist motion and press upwards onto the book to prevent it from moving. Again - there are lots of forces being applied here - but no energy is being produced or consumed in the process because nothing is moving (technically: because nothing is accelerating). The book will stay there, resting on the table forever without the table or the book "running out of energy" because they are doing that. But if you exert chemical energy from your muscles to pick up the book - you are giving it energy because you are moving it against the force of gravity...and when you drop it onto the floor, this energy will be released as it falls, turning into kinetic energy (motion) and then into heat and sound as it hits the floor. So just as we can't get "free" energy from gravity - so we also can't get it from magnets.
