Answer:
Of these things - I have two. One for me - and one for you. And when you ask about the price I simply smile and nod twice.
I run up and down the stairs without moving. What am I?
Feed me and I live. Give me water and I die. What am I?
By myself, I'm practically nothing. Worthless as a leader, but when I follow a group, their strength increases tenfold. What am I?
I am perfect with a head perfect without a head; Perfect with a tail perfect without a tail; Perfect with either neither or both.
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?