Answer:
There are several different kinds, but the one you pick doesn't do its job. What is it?
Come up and let us go. Go down and here we stay.
What is the heart of many risks, has a heart, but is not a living being?
Without a bridle, or a saddle, across a thing I ride a-straddle. And those I ride, by help of me, though almost blind, are made to see.
Feed me and I live, give me drink and I die.
What covers its face with its hands, speaks no language, yet most known what it's saying?