Answer:
I can run but not walk. Wherever I go thought follows close behind.
I am a protector. I sit on a bridge. One person can see right through me, while others wonder what I hide. What am I?
I am the only word that looks the same when spelled upside down.
I transfer oxygen from the atmosphere to your blood
I only exist when you are here. Where you never were I can never be.
What is lighter than what it is made of?