Engine

In their attempt to accomplish their own bidding, people use tools.
From the simplest – a crow bar, for example – to the more and more complicated ones.
The engine is one of the most interesting tools invented by us. While all the tools we used before were mere extensions of our hands, engines directly transform energy into movement. We, the operators, only control them. And provide them with energy.

But don’t have to transform that energy into movement ourselves. As we do when shoveling coal to feed a stove. Or to stoke a steam engine…

Wait a moment! You said we “don’t have to transform that energy into movement ourselves”. Then ‘we have to shovel coal to stoke the steam engine whenever we need that steam engine to work for us’.

Yep. An engine is still a tool. Still an extension of ourselves. And we still have to move our limbs – spend some energy – when operating one. The main difference between a hand shovel and a backhoe being the fact that all the energy needed to ‘activate’ the hand shovel flows through our muscles while most of the energy used to move the backhoe comes from its engine.
Another example being a horse drawn cart. We can carry things on our back – using exclusively the energy provided by our muscles – or we can load those things onto a horse drawn cart. Then drive it, channeling horse-muscle energy into the process – where ever we need those things to arrive.

Savvy?