Why don’t perpetual motion machines ever work?

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?

Enter one or more laws of thermodynamics—‘thermo’ meaning heat and ‘dynamics’ referring to work—which are violated by this idealized concept. This TED Ed lesson by Netta Schramm explains the conundrum, and shares a few creative examples of these hypothetical machines.