When it’s properly filled,
its contents are constantly spilled.
Some people really don’t like this riddle, so there’s a change you might not as well. But I think it works. In an attempt to steer you to the answer, focus on the last two lines as opposed to the first two. The first two are so riddle-y, they’re nearly hopeless to help you!
Your heart or your mind? If one of those aren’t the answer I think it still relates in a cool sort of way.