You fill it and it empties, A metaphor for plenty, Depending on what half you see, It's either full or empty
Makes apples fall from trees
What is used by man, tossed by trees, everywhere but unseen?
If you drop me I'm sure to crack. But give me a smile and I'll always smile back.
Dry though I may be, I am still a sea.
Big as a saucepan, deep as a cup, Even a river can't fill it up. What is it