Answer:
Once you know it exists, it no longer exists. What is it?
What must you give before you can keep it?
I'm the start of eternity and the end of space. There are two of me in heaven and one in hell.
They are two brothers. However much they run, they do not reach each other.
What always comes into the house through the keyhole?
I run through hills. I veer around mountains. I leap over rivers. And crawl through the forests. Step out your door to find me.