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:
- Filtered perception
- Being able to start practicing right away
- Sources of our inspiration
- Rip that ego apart
- Relative and absolute truth
- It’s all a matter of motivation
- The decision to follow a spiritual path
- Three higher trainings
- Opposite direction to dharma
- No substitute for being guided by a guru
- Maintaining a strong grip on the habits
- Merely the product of your own perception
- Dharma is not a therapy
- Our fundamental problem
- Sadness
- Mara’s five arrows
- Nothing genuinely works in samsara
- Mind-made illusions
- It cannot be fixed
- Very little time left for practice