Answer:
Once you know it exists, it no longer exists. What is it?
What is in seasons, seconds, centuries, and minutes, but not in decades, years, or days?
Never ahead, ever behind, yet flying swiftly past; For a child I last forever, for adults I'm gone too fast.
If you need a night out, call one of these over to take the little ones off your hands for a while.
The part of the bird, that is not in the sky, Which can swim in the ocean and always stay dry. What is it?
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.