If What Goes Up Must Come Down, What Comes Down That Must Go Up?

Riddle: If what goes up must come down, what comes down that must go up?

Answer: The Sun.

Source: https://www.riddles.com/index.php/1194

If what goes up must come down, what comes down that must go up? — If what goes up must come down, what comes down that must go up? by Anonymous v1.

Riddle: If what goes up must come down, what comes down that must go up? Answer: The Sun. — by Anonymous v2.