Answer:
What five-letter word becomes shorter when you add two more letters?
Upon me you can tread, though softly under cover. And I will take you places, that you have yet to discover. I'm high, and I'm low, though flat in the middle. And though a joy to the children, adults think of me little.
There are millions of me. I am very small but when moving fast I am deadly.
Round as a button, deep as a well. If you want me to talk, you must first pull my tail.
I pass before the sun, but make no shadow.
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