Riddle

I can be full, but I will never spill. I disappear at times, but return I always will. What am I?

Choose an answer

A.A cup
B.A balloon
C.A tank
D.Moon
Hint: It's a word with 4 letters, starting with "M".

Answer

Moon

Why this is the answer

The moon can appear fully round ('full') but never actually spills anything, and it seems to vanish during its new-moon phase before always returning.

This riddle is filed under "What Am I?" Riddles and rated Medium.