Most-viewed video of 2017: Why don't perpetual motion machines ever work? #10YearsofTEDEd* Perpetual motion machines — devices that can do work indefinitely without any external energy source — have captured many inventors’ imaginations because they could totally transform our relationship with energy. There’s just one problem: they don’t work. Why not? *We're counting down to our 10th birthday on March 12 by sharing ten of our most-viewed videos —one published in each year that we’ve been aroun