Answer:
Without this, eating food and Steven Spielberg's career would be very different.
My first master has four legs, my second master has two. My first I serve in life, my second I serve in death. Tough I am, yet soft beside. Against ladies cheeks I often reside.
They belong to me; they belong to you; They can make you feel happy or make you feel blue; They never end until the day you do.
My life can be measured in hours, I serve by being devoured. Thin, I am quick. Fat, I am slow. Wind is my foe.
I am a box that holds keys without locks, yet they can unlock your soul.
What covers its face with its hands, speaks no language, yet most known what it's saying?