Applied Knowledge or Primitive Man

You flip the switch and the lights come on so you can see after dark.  You don’t know how it works; it just does.  You open the refrigerator door and inside is meat and cheese, even though you know nothing about electricity, refrigeration, hunting or farming.  You turn the key and the engine comes to life so you can travel long distances over roads and bridges you don’t know how to build.

Producers used specialized applied knowledge to create useful things for others.  What do you produce for others that they can use even though they don’t know how it works or how do it themselves?

What knowledge do you possess and apply?