Answer:
It cannot be seen, it weighs nothing, but when put into a barrel, it makes it lighter.
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.
What word in the English language uses all five vowels plus Y in alphabetical order, and uses each only once?
I hide but my head is outside.
I can run but not walk. Wherever I go, thought follows close behind.
They are many and one, they wave and they drum, Used to cover a state, they go with you everywhere.