Answer:
Once you know it exists, it no longer exists. What is it?
A squiggly line tells you when you've done it wrong
There is one in every corner and two in every room.
I am a king but also a common device of measure. What am I?
What cannot talk but will always reply when spoken to?
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.