Answer:
When one does not know what it is, then it is something; but when one knows what it is, then it is nothing.
I usually wear a yellow coat. I usually have a dark head. I make marks wherever I go.
Shorter than my four siblings, but easily the strongest, Sometimes I wear a funny hat.
4 rubber ducks were floating in the bathtub, 2 floated away and 2 drowned. How many ducks are still alive?
What speeds on a track but isn't in a race and has a whistle to announce its steady pace?
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?