Garden Waterfall / Hydroelectric Micro-Generator Idea
 

Yep, that's a perpetual motion machine and humans have been trying to
make one of those for a few thousand years. It's too bad it won't work
because we would no longer need fossil fuels, we would all have
waterfalls in our front yard with a generator hooked up to it. What
you're talking about has actually been tried before as well, do a
Google search on perpetual motion machines and have a look at some
other ideas that didn't produce. Some of them look like they may work
but like Naomi said, you can't get more energy out than you put in.
Todd