Answer:
I am free for the taking. Through all of your life, though given but once at birth. I am less than nothing in weight, but will fell the strongest of you if held.
Until I am measured I am not known, Yet how you miss me when I have flown.
You eat something you neither plant nor plow. It is the son of water, but if water touches it, it dies.
What goes in, but never through? I've got some and so do you. We might lend one, or even two, but only if our trust is true.
If all Wibbles are Criggles, all Borkins are Kwumblins, no Hoggles are Borkins, and all Criggles are Borkins, is it true that all Borkins are Criggles?
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.