A buddy of mine invented one!

Really. It is really easy to make, he just hasn't figured out how to capture the energy yet. Here is how it works: First you butter a piece of toast. Then you get a cat and strap the toast, butter side up on the back of the cat.

As you know toast always lands butter side down and cats always land on their feet.

Then you throw the cat out the window, about a foot from the ground the cat/toast contraption will start spinning, toast side down, legs side down. It will keep spinning. Now the trick is how to get this contraption to transfer energy, how to keep the cat fed and the toast from drying out.

Don't try this at home, my friend is a trained professional.