Once a professor from the university went to a Zen master,
to inquire about Zen.
The master received the professor politely and invited him for a cup of tea.
The professor sat down and the master served the tea.
He poured his visitors cup full, and then kept on pouring...
The professor watched the overflow, untill he no longer could restrain himself:
'It is overfull ! NO more will go in !'
'Like this cup', the master said, 'you are full of your own thoughts, opinions and speculations.'
'How can I show you Zen, unless you first empty your cup!'