This tale comes from a book of Zen Buddhism stories. It is only one of many and in the following weeks and months I will add more, which I think have a nice meaning.
I will again point out that this is a fragment of Zen Buddhism. There are over 300 books on the subject available from eBay, amazon or mainstream book shops. If though, you are not that interested, just read, enjoy and share this one:
Naan Inn, a Japanese master during the midge era, received a university professor who came to inquire about Zen. Naan Inn served tea. He poured his visitors cup full, and then kept on pouring. The professor watched the overflow until he could no longer restrain himself, “it is over-full, no more will go in.” “Like this cup,” Naan Inn said, “you are full of your own opinions and speculations. How can I show you Zen unless you first empty your cup!?”