A Cup of Tea
Nan-in, a Japanese master during the Meiji era (1868-1912), received a university professor who came to inquire about Zen. Nan-in served tea. He poured his visitor's cup full, and then kept on pouring. The professor watched the overflow until he no longer could restrain himself. "It is overfull. No more will go in!" "Like this cup," Nan-in said, "you are full of your own opinions and speculations. How can I show you Zen unless you first empty your cup?"
Being a student is hard. Being a mentor is even harder.
The amount of effort, generosity, endurance and patience people need in order to be in a teaching position is astronomical these days. It is in the end a very selfless dedication that is needed in order to continue teaching.
The students tend to be the complete opposite of the teacher. And usually will have a hard time to understand the koan above. In this day and age of constant phone notifications, fidget spinners and constant interruptions, most people find it impossible to empty their mind, a Tabula Rasa so to speak.
Once you have achieved this, everything is possible, even the impossible, it just takes longer.