Answer:
Small, containing light, You'll need it in the dark, It will provide that spark, and shine into the night. Will light up any pyre, 'Cause it can help make a fire.
Prior to smartphones, people used this to record their lives.
When a doctor hits you with a hammer he is testing this
What is yours but only used by others?
What goes up and down the stairs without moving?
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?