You do not wash the cup, you wash the dirt; if you were to wash the cup, it would disappear completely. So, it is the dirt that is washable and has nothing at all to do with the cup.
This example is quite a good illustration of one of the most profound theories of the bodhisattvayana: we all have the potential to become Buddhas because we have Buddha nature. The problem is we have yet to realise it.

Dzongsar Khyentse Rinpoche
from the book Not for Happiness: A Guide to the So-Called Preliminary Practices
Read a random quote or see all quotes by Dzongsar Khyentse Rinpoche.
Further quotes from the book Not for Happiness:
- Relative and absolute truth
- Rip that ego apart
- The decision to follow a spiritual path
- It’s all a matter of motivation
- Three higher trainings
- No substitute for being guided by a guru
- Opposite direction to dharma
- Merely the product of your own perception
- Maintaining a strong grip on the habits
- Our fundamental problem
- Dharma is not a therapy
- Mara’s five arrows
- Sadness
- Mind-made illusions
- Nothing genuinely works in samsara
- Very little time left for practice
- It cannot be fixed
- What Is Bodhichitta
- We must also practice it
- Everything we experience is a product of mind