Answer:
When it comes to me you go on red and stop on green.
I'm not a bird but I can fly through the sky. I'm not a river but I'm full of water.
The more you add the less it weighs. What is it?
What has one eye but cannot see?
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.
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?