To start out with - they don't exist so lets not discuses that.
What does exist are these small machines that can run with extremely little friction which I would love to have here on my desktop - my only problem is that I CANNOT find them and I have forgotten what they where called - so maybe you guys can help.
What I am talking about are machines based on the most simple mechanics at all, such as collisions (newton cradle), rotations and oscillation motions (e.g. pendulums) with very low energy loss. They where kept alive with help for a battery but at an extremely low effect. They where quite stylish and I remember how fascinated is was at child looking at these sometimes sculpture-looking machines.. very simple and stylish models showing off the Newtonian world.. my guess is that they are also quite EXPENSIVE..!
Do you know what I am talking about - if yes please help me out..